JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, A00K03, doi:10.1029/2010JA016103, 2011
Laboratory studies of UV emissions from proton impact
on N2: The Lyman‐Birge‐Hopfield band system
for aurora analysis
Joseph M. Ajello,1 Rao S. Mangina,1 Douglas J. Strickland,2 and Dariusz Dziczek3
Received 9 September 2010; revised 14 December 2010; accepted 3 January 2011; published 19 April 2011.
[1] We have measured the emission cross sections of the Lyman‐Birge‐Hopfield (LBH)
a 1Pg − X 1S+g band system and several atomic nitrogen (N I) multiplets (1200, 1243,
1493 Å) by H+ (proton) impact on N2 over an impact energy range of 1–7 keV. The peak
proton‐impact‐induced emission cross section of the LBH band system (1260–2500 Å)
was measured to be 5.05 ± 1.52 × 10−17 cm2 at 7 keV. To the best of our knowledge,
the present LBH emission cross sections are reported for the first time in the far ultraviolet
(FUV) wavelength range of 1100–1600 Å. The proton energy range in this study,
when coupled with previously published 10–100 keV proton excited emissions of N I
multiplets, provides a wide energy range of emission cross sections for proton energy
loss transport codes. This energy range includes the peak cross section and the energy
range for Born scaling. The reported measurements lead to an important component
of monoenergetic yields for proton FUV auroral emission. Such yields, based on emission
cross sections and transport modeling, allowed for convenient comparison of emission
efficiencies between proton and electron aurora. In addition, we have measured the
H Ly a, LBH, and N I multiplet emission cross sections for H+2 and H+3 ion impact on N2
at 5 keV and found that the magnitude of H Ly a emission cross section, sem(Ly a),
follows in the order of impact ion mass H+3 > H+2 > H+.
Citation: Ajello, J. M., R. S. Mangina, D. J. Strickland, and D. Dziczek (2011), Laboratory studies of UV emissions from proton
impact on N2: The Lyman‐Birge‐Hopfield band system for aurora analysis, J. Geophys. Res., 116, A00K03,
doi:10.1029/2010JA016103.
1. Introduction
[2] Earth, Titan, Triton, and Pluto have nitrogen‐bearing
atmospheres in our solar system. Their emergent far ultraviolet ((FUV) 1200–2000 Å) airglow and auroral spectra
contain a wealth of information on solar‐driven and particle‐
driven processes involving N2. The only way to get a global,
instantaneous picture of the energetic particles input over
the auroral oval is through spectral imaging. Imaging the
proton aurora is an excellent probe for investigating magnetospheric substorms and magnetosphere‐ionosphere coupling processes [Rees, 1989; Samson et al., 1992; Deehr
and Lummerzheim, 2001; Immel et al., 2002; Mende et al.,
2000]. For instance, atomic H emissions resulting from
excited H atoms produced within the incident proton beam
are a unique signature of proton precipitation. The Lyman‐
Birge‐Hopfield (LBH) band is one of the most important
molecular emissions of excited nitrogen in the FUV. It is
always found and can be produced due to impact excitation
1
Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA.
2
Computational Physics, Inc., Springfield, Virginia, USA.
3
Institute of Physics, Nicolaus Copernicus University, Torun, Poland.
Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2010JA016103
of protons and electrons in the aurora and airglow observations of nitrogen‐bearing atmospheres of Earth [e.g.,
Bishop and Feldman, 2003; Strickland et al., 2001], Titan
[e.g., Ajello et al., 2008; Sittler et al., 2009] and Triton [e.g.,
Broadfoot et al., 1989] and is expected at Pluto [Stern
et al., 2008].
[3] Recently, NASA’s Imager for Magnetopause‐to‐Aurora
Global Exploration (IMAGE) spacecraft globally imaged
proton aurora for the first time and made a remarkable set
of observations, leading to a breakthrough in understanding
the origin of a peculiar and puzzling type of aurora, seen as
bright spots in the Earth’s atmosphere and called “dayside
proton auroral spots” [Frey et al., 2002]. They are now
known to occur when fractures appear in the Earth’s magnetic
field, allowing particles emitted from the Sun to pass through
and collide with molecules in the atmosphere. This result
has opened up a new area of research to observe dayside
proton aurora, and use those observations to know where and
how the cracks in the magnetic field are formed and how long
the cracks remain open. That makes it a powerful tool to study
the entry of the solar wind into the Earth’s magnetosphere.
[4] The 2003 launch of the Defense Meteorological Satellite Program (DMSP) F16 satellite, which contains, among
its suite of sensors, Special Sensor Auroral Particle Sensor
(SSJ/5) and Special Sensor Ultraviolet Spectrographic Imager
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(SSUSI) [e.g., Paxton et al., 1992a, 1992b] made it possible,
for the first time, to undertake extensive statistical studies
of auroral output at far ultraviolet (FUV) wavelengths as a
function of auroral input of energetic electrons and ions
[Knight et al., 2008; J. T. Correira et al., A downward revision of a recently reported proton auroral LBH emission
efficiency, submitted to Journal of Geophysical Research,
2010]. Independently, FUV can also be used to infer the solar
energy input [e.g., Strickland et al., 2007] and also determine
characteristics of auroral particles that deposit energy in the
thermosphere [e.g., Strickland et al., 1993; Germany et al.,
1994a, 1994b; Lummerzheim et al., 1991].
[5] As observed in many satellite missions (e.g., by the Global
Ultraviolet Imager (GUVI)) aboard NASA’s Thermosphere
Ionosphere Mesosphere Energetics and Dynamics (TIMED)
[e.g., Christensen et al., 2003], FUV on IMAGE [Mende
et al., 2000], SSUSI on DMSP, High Throughput Imaging
Echelle Spectrograph (HiTIES) [McWhirter et al., 2003], and
the Ultraviolet and Visible Imaging and Spectrograph Imaging
(UVISI) on the Midcourse Space Experiment (MSX) [e.g.,
Strickland et al., 2001] the auroral emission spectrum spanning
the FUV to visible region is dominated by (1) N2 LBH (within
1000–2000 Å), LBHS (in the shorter wavelength range 1400–
1530 Å), and LBHL (in the longer wavelength 1650–1800 Å);
(2) H Ly a (1216 Å), Ha (6563 Å), and Hb (4861 Å); (3) O I
multiplets (989, 1027, 1304, 1356 Å); (4) first positive band
(B 3P+g → A 3S+u ) heads of N2 at 6545 Å, 7534 Å, N I lines
(1100–1750 Å), first negative band (B 2S+u → X 2S+g ) heads
of N+2 at 3914 and 4278 Å, and N2 second positive band
(C 3Pu → B 3Pg) of N2 at 3371 Å. It is known that energetic
protons and electrons do not interact in the same way within
the atmosphere [Basu et al., 1993; Strickland et al., 1993;
Galand et al., 1997; Galand and Richmond, 2001; Galand
and Lummerzheim, 2004; Knight et al., 2008], one difference being higher LBH emission efficiency for proton aurora.
While current usage of cross sections leads to ∼50% higher
efficiency, Knight et al. found even greater values with some
downward revision recommended by Correira et al. (submitted manuscript, 2010) (also see section 4). Therefore, it
is crucial to separate the electron and proton components
of the auroral precipitation in order to correctly assess ionospheric conductivities, heating, and composition changes.
This is one of the important scientific issues of polar aeronomy, as stated by Coupling, Energetic, and Dynamics of
Atmospheric Regions (CEDAR)/Phase III. In previous proton workshops (1994, 1999), it was realized that many of the
available excitation cross sections for these auroral emissions
due to proton‐impact from their thresholds to 40 keV (i.e.,
typical average peak energy observed in the proton aurora)
are still poorly known or undetermined and thus become
a limiting factor for accurate determination of charged and
neutral particle interactions with species in Earth’s and outer
space atmospheres.
[6] Much of current knowledge about the relative
importance of proton versus electron precipitation has come
from particle measurements and their use in developing
statistical models. Extensive use has been made of DMSP
SSJ/4‐based models by Hardy et al. [1985, 1989] (electrons
and protons). These models illustrate a displacement of the
statistical ovals from one another that can result in pure or
nearly pure proton precipitation in the vicinity and south of
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the electron equatorial boundary and most prevalent during
premidnight hours. The Space Environment Monitor (SEM)
on NOAA POES and TIROS satellites [Evans and Greer,
2000; Raben et al., 1995] complements SSJ/4 with measurements to much higher energies (to 100s of keV in contrast
to an upper limit of 30 keV for SSJ/4). Such measurements
demonstrate that significant energy flux can reside above
30 keV during active times [e.g., Coumans et al., 2002]. Maps
of proton precipitation similar to the Hardy maps but for
higher energies may be seen by Fang et al. [2007] (SEM on
POES) and by Codrescu et al. [1997] (SEM on TIROS).
[7] LBH emissions result from collisions of N2 with
energetic electrons, combined neutral H and protons, as well
as from secondary electrons produced by ionizing events
[Knight et al., 2008]. Knight et al. [2008] reported unexpectedly high efficiencies of LBH emission for proton precipitation, higher than predicted by current models. The
earliest measurements of LBH excitation by H+ impact on
N2 were obtained by energy loss spectroscopy (from 20 to
120 keV by Schowengerdt and Park [1970] and from 0.6 to
4 keV by Moore [1972]). As shown by Moore, the comparison of excitation cross sections of these two measurements do not agree in magnitude so as to join them together
to form a single cross‐section curve for obtaining missing
cross‐section values. At the present time, the continued
absence of measured proton impact emission cross sections
for the LBH band system of N2 and the need of these data
for modeling prompted this laboratory study of LBH and N I
emission cross sections of N2, which is complementary to
our ongoing studies of electron impact emissions of species
of interest to Earth and outer space atmospheres [Ajello
et al., 2010]. Our Jet Propulsion Laboratory (JPL) team
recently remeasured the cross sections for electron excited
LBH emissions of N2 more accurately [Young et al., 2010]
and revised our earlier measurements of Ajello and
Shemansky [1985, hereafter AS85]. It is important to compare peak electron and proton emission cross sections in the
energy regions where auroral particles have high‐energy
fluxes. The energy range of interest for proton impact
fluorescence extends from ∼1 keV to ∼100 keV. The key
to the energy range is to get to high enough energies to
measure beyond cross‐section maxima to the Born region
(∼100 keV), so cross sections can be reasonably estimated
using the Born approximation to a few hundred keV. We
make a first step in this program by measuring the LBH
cross sections from 1 to 7 keV, the region of the emission
cross‐section maximum. We measured and analyzed the
UV spectrum, which is calibrated by standard techniques
developed in this laboratory [Liu et al., 1995]. The absolute
emission cross sections of the LBH band system show
strong efficiency of proton excitation of LBH band system
compared to electron impact LBH emissions.
[8] In this paper, we describe our new laboratory facility
in section 2: (1) ion source, (2) collision chamber wherein
the proton‐neutral (i.e., N2) gas collisions occur orthoganally in a crossed‐beams geometry, and (3) UV spectrometer
with a detector observing the fluorescence on the third
perpendicular axis. We present the calibrated spectral intensities in the FUV as a function of proton energy to determine
emission cross sections in section 3. We discuss the application of the results to the aurora and summarize current
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Figure 1. The experimental system comprising a Coultron ion beam source, an electrostatic Kimball
electron gun (EG5), and a collision chamber designed to cross the proton (or electron) beam with a beam
of N2 from a capillary array source. The projectile (hydrogen ions or electron) beams and the input axis of
the UV spectrometer are all perpendicular to the N2 gas beam. The ion beam is at 90° to the spectrometer
axis and at 36° to the electron gun axis.
knowledge of H+ + N2 emission cross sections over the
energy range of 1–100 keV in section 4. Finally, we summarize in section 5.
2. Experiment
[9] We have built a state‐of‐the‐art crossed‐beams apparatus that can be used for systematic emission studies of
either proton‐atom/molecule or electron‐atom/molecule collisions. A schematic of the present experimental arrangement of the crossed‐beams apparatus is shown in Figure 1.
The apparatus mainly consists of two high vacuum chambers,
a high‐energy resolution electron source, a Colutron ion
source, an Acton 0.2m UV spectrometer, two Faraday cups
(one for electron beam and the other for ion beam current
measurement), and a capillary array to flow in target gas
beam. The capillary array of N2 source, comprising a bundle
of 50–100 holes 8 ± 3 microns in diameter with an aspect
ratio of 6–10:1, was manufactured by Lenox Laser Corporation. The main vacuum chamber (0.35 m diam × 0.25 m
height) is pumped by a combination of Varian V‐3KT 2000 l/s
turbo molecular pump and dual stage 24 ft3/m rotary vane
pump (Varian SD 700), and Colutron ion source chamber is
pumped separately with a 500 l/s Varian 550 turbo molecular
pump backed by an oil sealed mechanical pump. A base
pressure of 2 × 10−7 torr in the main chamber and 1 × 10−9 torr
in the ion source chamber is attained within a couple days
of pumping. Since the mass selection aperture at the end of
the ion source separates the two vacuum chambers (see
section 2.1), differential pumping of ion chamber is required
to maintain a vacuum of better than 5 × 10−6 torr in the ion
source region while flowing the target gas into the main collision chamber (∼3 × 10−5 torr). Measurements of the fluorescence induced by ion‐ or electron‐molecule collisions are
controlled by a personal computer (PC) executing custom
software developed in our laboratory (see section 2.2).
2.1. The Ion Source
[10] The Colutron ion source, which can be baked up to
200°C, consists of a DC discharge cell, heat sink, an ion
acceleration electrode, an Einzel lens for focusing the ion
beam, a E × B velocity filter for selection of a desired
projectile ion (e.g., H+ or H+2 or H+3 ), a set of beam steering
electrodes, a beam‐transporting cylindrical electrode with a
rectangular exit aperture at its end, and finally a decelerator
that allows provision for ion beam energies from 10 eV to
500 eV. The gas discharge cell contains a tungsten filament
and an anode with a 20 microns aperture for ion extraction,
which are encapsulated in a quartz minichamber with an
inlet for H2 gas to flow into the cell through a variable leak
valve. The velocity filter consists of a magnet, a pair of
electrostatic deflection plates, and a set of shims. The electric
field plates are mounted between the magnet poles to produce a uniform electric field (E) perpendicular to the magnetic field (B) to achieve a desired E × B configuration.
The shims are wired separately so that each one’s voltage is
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adjusted independently enabling the electric field in the filter
so as to achieve cylindrical symmetry for optimum collimation of the proton beam. The magnetic field is provided
by two electromagnets with combined maximum field of
0.3 T. High magnetic and electric fields increases the dispersion of the ion beam and improves the resolution. In the
hydrogen discharge, three types of ions, H+, H+2 , H+3 , coexist
with different concentrations based on the pressure in the cell
and electric field configuration. In our measurements, we
found their fluxes in the sequence of H+2 > H+3 > H+, based on
total Faraday cup current. As long as the dissociation degree
is limited, the H+2 ion production in the discharge cell is
consistently higher than of the other two ions, and the formation of H+3 occurs mostly due to exothermic nonresonant
charge exchange reaction by H+2 + H2 → H+3 + H. For a
chosen ion kinetic energy Ei, the accelerated ion beam contains all three ions but with different velocities as defined by
v = (2qEi/M)1/2, where M is the ion mass with charge q,
velocity ratios H+/H+n of 1.0, 1.41, 1.73 for H+n = H+, H+2 , and
H+3 , respectively. The velocity filter acts as a mass filter for
ions with equal kinetic energy. We observed that the focusing
property of the velocity filter has very little effect with the
change in magnet current as compared to the change in
electric field. Beam characterization was done by keeping
the electric field of the velocity filter constant and scanning
the magnet current. Voltages on a set of beam‐focusing and
beam‐steering electrostatic lenses placed before the velocity
filter were adjusted to maximize the ion beam transmission
through the ion beam transporting 8″ long cylindrical electrode (grounded guard tube), and focus onto a scintillating
beam collector screen placed at the entrance of the Faraday
cup. We observed three beam spots of H+, H+2 , H+3 ions at
a distance of about 10 mm apart from each other on the
screen. After optimum tuning of the ion gun electrical and
magnetic settings for maximum beam currents of well separated hydrogen ions, we designed a suitable mass selection
aperture (3 mm width × 10 mm height) and placed it at the
end of the cylindrical electrode just before the ion‐molecule
interaction region. The beam spot of about 3 mm for each ion
was obtained after dispersion. The present size of the mass
selection aperture is sufficient to block the paths of all other
unwanted ions that are deflected off axis by the velocity filter.
The ion beam current is measured with a programmable
Keithley Source Measure Unit (Model 2612A) controlled by
data acquisition software (see section 2.2). It also provides
the bias voltages of the Faraday cup and secondary electron
suppressor necessary to ensure that the primary beam current
is fully accounted and no secondary electrons or ions from
Faraday cup enter into the interaction region. Without the
decelerator section of the ion source, the projectile ion energy
can be varied from 0.1 to 10 keV with beam currents of 0.2
to 10 mA depending on the ion selection and its energy.
2.2. The Data Acquisition System
[11] Experimental data collection system is managed by
a PC running custom software developed using National
Instruments programming environment, which controls and
monitors important experimental parameters and performs
fluorescence data acquisition. The data acquisition system
(DAS) performs repetitive wavelength scans to minimize
systematic errors associated with long‐term drifts in the
experimental conditions.
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[12] Wavelength positions were changed in small increments by rotating the spectrometer’s diffraction grating
driven by a stepper motor with a microstepping controller/
driver (SilverMax, QuickSilver Controls, Inc.) directly controlled by the data acquisition software. The accuracy of the
positioning (better than 0.02 Å) significantly exceeded the
requirements following from the selected spectral resolution
of the measurements. At each preselected stationary wavelength position, the fluorescence photon signal from the ion
or electron interaction with the target molecule is recorded by
counting pulses produced by photon detector with a digital
counter for a fixed dwell time. The photon counter and the
accurate gating dwell timer (50 ns resolution) are formed of
counters, a time base generator and digital outputs of a
National Instruments PCI‐6602 Counter/Timer board (with
use of simple external fast logic gates).
[13] DAS also monitors and records data on the most
crucial experimental parameters such as the background
pressure in the collision chamber and the proton beam current
collected with the two‐element Faraday Cup. The chamber
pressure is related to the number flux of target molecules
effusing from the capillary array in the crossed‐beams mode
and to the target gas number density in the swarm mode.
Recording of the chamber pressure facilitates accounting for
the variation of the density of target molecules in the region
of interaction in given experimental conditions. A Bayard‐
Alpert ion gauge controlled by a Varian XGS‐600 unit is used
to monitor the pressure. Similarly, recorded instantaneous
values of the ion beam current make possible accounting
for its fluctuations at the stage of data analysis. The emission
spectra accumulated over a number of scans are corrected
for long‐term drifts, background signal contribution, and
intensity calibration of the spectrometer (see section 2.3) at
the stage of data analysis.
2.3. The Optical System UV Calibration
[14] For calibration of the crossed‐beams apparatus and
UV spectrometer, a six‐element electrostatic electron gun
of Kimball Physics Model‐5A is used. The performance of
the present high‐energy resolution electron gun was described
by James et al. [1997]. In the present crossed‐beams experimental arrangement, an energy selected electron beam collides
orthogonally with a target gas beam of molecules effusing
through the capillary array into the collision chamber at a
constant flow rate controlled by a Varian variable leak valve.
With gas flow into the collision region, the main chamber
pressure raised 2 orders of magnitude from 10−7 to 10−5 torr
and the ion source chamber pressure by 3 orders from 10−9 to
10−6 torr. The background N2 gas pressure was maintained at
∼3 × 10−5 torr during the measurements. We have measured
the low‐resolution (4Å FWHM) UV electron‐induced fluorescence spectra under nearly optically thin conditions using a
0.2 m Acton VM‐502 UV spectrometer coupled to the collision chamber. The spectrometer is capable of operating at
resolving power of l/Dl = 200 at 1000 Å with a variety of
detectors from the extreme ultraviolet (EUV) to the near IR
(50–1200 nm). The single channel detector is a Hamamatsu
pulse‐counting R6836P photomultiplier tube with magnesium
fluoride window and 23–25 mm photocathode of cesium
telluride for wavelengths from 1150 to 3200 Å with less than
5c/s dark noise counts. The holographic grating used with this
spectrometer is a MgF2 concave 1200 grooves/mm grating
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Figure 2. The sensitivity of the spectrometer and photomultiplier optical system as a function of wavelength obtained as described by Liu et al. [1995].
with a horizontal aperture ratio of f/4.5. The fast optical system
results in a wide field of view of the collision region (∼10 deg
square). The UV photon signal is detected by the spectrometer
with an optic axis at 90° to the plane defined by the crossed
ion or electron beam and N2 target molecular beam. The
interaction volume is approximately 2 mm3. The relative
inverse sensitivity calibration data was obtained by measuring
the intensities of emissions from electron impact excitation of
H2 in the 1100 to 1700 Å range, and comparing them to the
modeled H2 intensities [Liu et al., 1995; Jonin et al., 2000;
Ajello et al., 2005]. The resulting calibration sensitivity and
inverse sensitivity data is shown in Figure 2. Peak sensitivity
occurs near 1200 Å, the blaze wavelength for the MgF2
concave 1200 grooves/mm grating. The curve was smoothly
fitted by a cubic spline to the measured calibration points
to correct the measured spectra. The calibration curve so
obtained was applied in correcting the measured fluorescence
spectrum from ion‐molecule collisions.
[15] There are two main spatial regions within the collision chamber that are responsible for exciting the H+‐N2
FUV spectrum. We show in Figure 3 a typical set of three
calibrated spectra at 5 keV proton energy that accompanied
each cross‐section measurement at the five energies studied:
1, 2, 3, 5 and 7 keV. We measure in the order: (1) a
background (shown in green in Figure 3), (2) a swarm
spectrum in black (background subtracted), (3) a crossed‐
beams spectrum (X beam) in red (background subtracted),
and (4) a “dot” spectrum that is the difference between the
cross beam and swarm spectra. The ratio of the photon
signals at H Ly a (and the LBH bands) in crossed‐beams
and swarm modes is 6, indicating that about 83% of the
signal in the crossed‐beams mode arises from the interaction
region and 17% from the background N2 gas. The relative
“crossed‐beams” and “dot” spectra are identical. The improve-
ment in signal‐to‐noise ratio (S/N) with a capillary array is
dramatic. The axial length of the proton beam within the
optical field of view of the UV spectrometer is 3 cm on each
side of the capillary array and the exit aperture distance
from the guard tube to the capillary is 5 cm. Within this path
length and with the low‐N2 gas number densities (number
densities < 1012 cm−3) present in both chambers we expect
insignificant conversion of the proton beam to a fast H beam
by electron capture collisions.
2.4. Emission Cross Section for H+, H+2 , and H+3
Impact on N2
[16] As discussed in sections 2.2 and 2.3, we measured the
fluorescence UV emissions of N2 induced by H+ impact at
ion energies of 1–7 keV and by H+2 , and H+3 at 5 keV in the
crossed‐beams collision experimental geometry. The crossed
beam emission spectrum was recorded by scanning the
grating from 1100 to 1600 Å at a spectral resolution of 4.0 Å
FWHM for the present experiment. The wavelength increment was 0.8 Å, and the integration time for each data point
was 5 s per scan, and the data was accumulated over a
number of scans, typically 20, for each wavelength range.
The signal was found to be linear with N2 background gas
pressure over the range of 0.1–5.0 × 10−5 torr. The polarization of the radiation is negligible for the N2 transitions at
1–10 keV excitation energy.
[17] When the hydrogen discharge cell was in operation,
the background gas pressure in the Colutron chamber
increased from 10−8 torr (with an N2 gas load in the main
chamber) to 10−6 torr due to neutral hydrogen gas leaking
out through the 20 micron aperture on the anode. Since only
the mass selection aperture mounted at the end of the long
cylindrical ion beam transporting tube separates the two
vacuum chambers, neutral hydrogen gas, which leaks out
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Figure 3. The data set comparison of three calibrated laboratory spectra from three modes of operation:
(1) background spectrum in green with the Colutron gas pressure of 5 × 10−6 torr and a collision chamber
gas pressure of 8 × 10−7 torr from H2, consisting of a single emission from H Ly a; (2) a swarm gas
spectrum in black at 3.3 × 10−5 torr; (3) a crossed‐beams (X beam) spectrum in red at 3.3 × 10−5 torr;
and (4) a “dot” spectrum in blue that is the difference between the crossed‐beams spectrum and swarm
spectrum and represents the emission from the interaction volume only. Each spectrum was measured at
1.5 mA Faraday cup proton current and a Colutron gas pressure of 5 × 10−6 torr. The crossed‐beams spectrum and swarm spectrum were each modified by the subtraction of the background spectrum in green.
through the anode aperture, can easily flow into the main
chamber. This results in weak H Ly a background emission
from ion + H2 collisions as shown in Figure 3, which
contaminates the emission from ion + N2 collisions. In order
to subtract the background contribution from H and H2
emissions to the N2 emission spectrum, we measured the
emissions due to ion collisions with background H2 in the
main chamber for all three H+, H+2 , and H+3 projectile ions
in the same wavelength range without flowing N2 into the
main chamber. We found no evidence for Lyman and
Werner bands above the background level. We observed
that the background contribution to the total signal comes
only from H Ly a emission due to ion + H2 collisions, and
we subtracted the background signal from the N2 spectrum
as discussed in sections 2.2 and 2.3.
[18] We show in Figure 4 a comparison of the proton
impact spectra of N2 in the swarm and crossed‐beams mode.
The calibrated spectra of e−, H+, H+2 , and H+3 impact on N2
are shown in Figures 5, 6, 7, and 8. We show in Figure 5
the proton‐induced fluorescence spectra of N2 at 7 keV
with identification numbers for each feature. We compare
electron impact and proton impact crossed‐beams fluorescence emission spectra in Figure 6. Finally in Figure 7 we
show emission spectra from H+, H+2 , and H+3 projectile
impact on N2; and in Figure 8 the emission cross sections
of the LBH band system, the NI multiplets and H Ly a as
a function of proton energy from 1 to 100 keV for this work
and other published results.
2.5. The LBH Glow Correction
[19] Figure 4 shows the emission spectra of H+ (5 keV) +
N2 → FUV observed in the crossed‐beams and swarm
modes. The latter thermodynamic state of the N2 gas arises
inside the chamber for a uniformly filled gas chamber at
300 K. Ideally, an infinitely long proton beam in a uniformly filled chamber will produce a cylindrically uniform
glow region, where the emission and excitation rates per
unit length of the beam are equal. On the other hand the
small interaction volume observed in the crossed beams
mode produces a nearly spherically symmetric glow pattern. The two proton impact fluorescence spectra shown in
Figure 4 are nearly identical and are an important result in
correcting for drift of excited a 1Pg state N2 molecules out
of the 10° wide field‐of‐view (FOV), since the cylindrically
symmetric glow pattern can be calculated analytically for a
given lifetime [Ajello, 1970]. The identification of the FUV
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Figure 4. A comparison of 5 keV proton‐impact‐induced fluorescence spectra in swarm and crossed‐
beams mode.
spectral features are discussed in section 3. Note that the
ratio of H Ly a (peak intensity and integrated) to the LBH
bands are the same. The difference in integrated intensities
between the LBH bands and the N I multiplets is 12%.
[20] One of the complicating factors for the shape of the
glow profile about a cylindrically symmetric (swarm mode)
is the long lifetime of the a 1Pg state, which leads to significant drift of the excited molecule away from the original
point of interaction. The a 1Pg state is metastable and
decays to the ground gerade state (X 1S+g ) via electric quadrupole and magnetic dipole transitions (v′ = 0 through 6 or
predissociates for v′ > 6). When viewing on center toward
Figure 5. A 7 keV proton‐impact‐induced FUV fluorescence spectrum with 25 identified features listed
in Table 1 with cross sections. The N2 background gas pressure was 3.4 × 10 5 torr, Faraday current was
1.5 mA, and discharge pressure was 300 m torr.
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AJELLO ET AL.: UV EMISSIONS FROM PROTON IMPACT ON N2
Figure 6. A comparison of a 5 keV proton‐impact‐induced fluorescence spectrum with a 100 eV electron‐
impact‐induced fluorescence spectrum measured, using the same experimental setup.
the optic axis directly at the proton beam the long lifetime of
the N2 (a 1Pg) state resulted in a significant portion of these
excited species will drift out of the FOV of the detector
before radiating. The minimum ray height of the glow at the
edge of the 11° vertical field of view is 2.8 cm radial from
the proton beam; and the minimum ray height of the glow at
the edge of the 13° horizontal field of view is 3.1 cm radial
from the proton beam. The loss of excited a 1Pg metastable
molecules from the field of view needs to be quantified to
provide absolute emission cross sections in this experiment.
Absolute electron impact emission cross‐section studies for
metastable species rely on a LBH emission glow model to
extrapolate signal outside the FOV. Two metastable species
have been studied by this method in our laboratory: the LBH
band system (AS85) from direct excitation of N2 and the
135.6 nm atomic O 5S emission [Kanik et al., 2003] from
dissociative excitation of O2. The lifetime of the a 1Pg state
has been, until now, uncertain, with experiments and theory
reporting values that typically range from 54 ms to 170 ms.
The uncertainty in lifetime combined can lead to considerable uncertainty in the absolute emission cross section using
the glow model. For example, for the proton apparatus used
in this experiment the percentage of LBH glow observed
could potentially vary between 51 to 84% for the range of
lifetimes from 55 to 170 ms.
[21] We have recently remeasured the glow profile in a
separate experiment to be described in a future publication
(J. M. Ajello, private communication, 2010) at the University of Colorado in the same apparatus that measured the
1356 Å glow profile of the O 5S state from dissociative
excitation of O2 [Kanik et al., 2003]. The lifetime was found
to be 55 ms within 30% in agreement with the work of
Marinelli et al. [1988]. We use that result to quantify the
correction to the crossed‐beams spectra studied here by
correcting the observed intensities by a factor of 1.16 to
account for emissions outside of the field‐of‐view. We
measured all spectra in the crossed beams mode and utilized
the same correction factor. The crossed‐beams mode is
preferential because of the approximate 6:1 improvement in
signal‐to‐noise ratio (S/N) over the swarm mode.
3. Results and Discussion
3.1. Proton Emission Cross Sections
[22] We have studied at low‐spectral resolution the proton‐impact‐induced fluorescence spectrum of N2 in order to
provide emission cross sections needed to model proton
aurora. The principal ion‐molecule processes we have
studied for emissions are:
Hþ ð17 keVÞ þ N2 ! Hþ þ N2 * a 1 Pg
! Hþ þ N2 þ hLBH ; by excitation
ð1Þ
Hþ ð17 keVÞ þ N2 ! Hþ þ N2 *ðrepulsiveÞ
! Hþ þ N þ N þ h
1200;1493A...:
; by excitation ð2Þ
Hþ ð17 keVÞ þ N2 ! Ho* þ Nþ
2
! Ho þ Nþ
þ
h
Ly
; by electron capture:
2
ð3Þ
[23] To obtain integrated emission cross sections for each
atomic and molecular feature, we have measured laboratory
spectra at five energies: 1, 2, 3, 5 and 7 keV. We show in
Figure 5 the calibrated spectrum at 7 keV proton impact
energy over the wavelength range of 1100–1600 Å. There are
25 identified features numbered from 1 through 25. We show
in Figure 6 for comparison a 100 eV electron‐impact‐induced
8 of 21
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Figure 7. An overview of H+, H+2 , and H+3 FUV fluorescence spectrum with 25 identified features listed
in Table 2 with emission cross sections.
9 of 21
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Figure 8. The FUV emission cross sections of N2 by H+ impact from 1 to 100 keV: JPL from 1 to 7 keV
and published 1–130 keV of the H Ly a; N I multiplets at 1200, 1243, and 1493 Å; and the N2 (a 1Pg →
X 1S+g ) LBH band system which extends from 1262 to 2600 Å on a log‐log plot. The published data
references can be found by Avakyan et al. [1998].
fluorescence spectrum from 1100 to 1600 Å with a 5 keV
proton impact fluorescence spectrum measured from the
same instrument. The two sets of features are identical except
for the H Ly a feature in the proton spectrum at 1215.7 Å.
All other features in both spectra are the prominent members
of the LBH band system and N I multiplets.
[24] The features in Figure 5 are identified in Table 1
along with the proton emission cross sections for 1–7 keV
normalized to the value of the H Ly a (2p‐1s) emission
cross section found in the review by Avakyan et al. [1998],
following a small needed correction to the measured intensity from cascade loss described below (similar to LBH
bands already discussed). Atoms in the metastable 2s state are
deactivated at the walls after they pass through the observation region. The review work of Avakyan et al. [1998] is
key to providing the absolute cross sections of this experimental study. The recommended cross sections of Avakyan
et al., include cascade, and represent a true emission cross
section, sem = sex + scascade. The Avakyan et al. recommended
H Ly a proton emission cross sections include direct
excitation of the 2p state and cascade from higher‐lying ns,
nd states (Balmer series emitting states with long lifetimes
> 15 ns for cascade excitation versus a short lifetime of 1.5 ns
for direct excitation). The distribution of H Ly a emission
cross‐section data points in the review by Avakyan et al.
summarizing the results of the various sets of experimenters
indicates an uncertainty of about 15% for sem.
[25] The recommended cross sections used in the review
by Avakyan et al. [1998] have been synthesized from several
authors and included corrections. For example, the H Ly a
proton emission cross sections for absolute calibration of the
emission cross section of the features in the 1–7 keV spectra
were measured by two sets of authors: Van Zyl and
Neumann [1988], who studied low‐energy collisions of H+
with N2, and by Birely and McNeal [1971]. Higher‐energy
emission proton cross sections beyond 10 keV are provided
by Dahlberg et al. [1967] for the Avakyan et al. review.
[26] Van Zyl and Neumann [1988] calculated H+ + N2
cascade cross sections, scascade, contributions from ns and
nd states to be nearly constant at about 20–25% of sem from
1 to 10 keV. Their earlier measurements of Ha and Hb were
the basis for separating the three separate Balmer series
components [Van Zyl and Neumann, 1980]. The H Ly a
cascade and emission cross‐section values are both given
in Table 1 for 1–7 keV. The uncertainties in this cascade
calculation are estimated to be [Van Zyl and Gealy, 1987] a
combination of about 15% in Ha and Hb cross sections and
another 10% in determining the separate ns,np → 2s,2p
emission components from models. On the basis of the work
of Van Zyl and coworkers we can conclude that cascade loss
does not contribute significantly to the observed H Ly a
intensity in this proton induced fluorescence experiment
over the path length of 2.8 cm (3.1 cm) (20 cm) from the
vertical (horizontal) (along optic axis) interaction region to
the edge of the field of view(s) or along the optic axis and
can be corrected to a first approximation. In this experiment
there is a correction of 10% to the measured H Ly a
intensity owing to the long lifetime of the H(ns, n > 2) states
(lifetime > 150 ns) produced by the process of electron
capture in equation (3), most of the H Ly a photons from
the ns→2p→1s Balmer series cascade excitation will escape
from the 5.6 cm (6.2) square FOV and 20 cm length along
10 of 21
1159.2
1172.8
1183.2
1190.4
1205.6
1223.2
1236.8
1266.4
1288.8
2
3
4
5
6
7
8
9
Feature Number
1
11 of 21
1297.6
1272.8
1243.2
1215.2
1224.0
1199.2
1188.8
1176.0
1168.0
Wave‐length Wave‐length
Start (Å)
Peak (Å)
1289.5–1290.33
1291.8
1298.25
1176.51
1176.53
1177.695
1188.971
1189.249
1190.494
‐
1190.019
1199.89
1200.45
1201.85
1215.7
1225.026
1225.368
1225.374
1228.407
1228.785
1228.791
1243.171
‐
1243.313
1273.2
1275.038
1276.19
1277.41
1159–1172
Wave‐length
Peak Theory/
Observed (Å)
1306.4
1280.0
1252.8
1223.2
1236.8
1205.6
1190.4
1183.2
1172.8
Wave‐length
End (Å)
NI,II
NI,II
b(1,11)
a(5,0)
a(6,0)
NII(3Do ← 3P)
b′(1,12)
c′4(0,12)
NI(g 4So ← 4P)
NI(g 4So ← 4P)
NI(g 4So ← 4P)
H Ly a
NI(2Po ← 2D)
NI(2Po ← 2D)
NI(2Po ← 2D)
NI(2Po ← 2P)
NI(2Po ← 2P)
NI(2Po ← 2P)
NI(2Do ← 2D)
NI(2Do ← 2D
NI(2Po ← 2,4P)
NI(2Do ← 4P)
NI(2Do ← 2P)
NI(2Do ← 4F)
NI(2Do ← 4P, 2F)
NI(2Do ← 2P)
NI(2Do ← 2P)
NI(2Do ← 2P)
NI(g 4So ← 4P)
NI(g 4So ← 4P)
NI(2Po ← 2,4P, 2,4F)
Wavelength
Identificationa
Total
5.24
LBH
3.62
Non‐LBH
1.62
Total
11.6
LBH
9.65
Non‐LBH
1.98
15.8
6.80
Total
4.95
LBH
3.42
Non‐LBH
1.54
Total
9.61
LBH
7.97
Non‐LBH
1.63
416.0/88b
26.8
186.0/40b
12.3
5.40
3.69
54.1
6.54
4.64
18.8
9.02
2 keV
Cross Section
(× 10−9) (cm2)
4.83
1 keV
Cross Section
(× 10−9) (cm2)
Table 1. The Proton‐Impact‐Induced Fluorescence Spectrum Identifications and Emission Cross Sections of N2 From 1150 to 1600 Å
Total
4.29
LBH
2.96
Non‐LBH
1.33
Total
11.0
LBH
9.14
Non‐LBH
1.87
21.5
498.0/101b
32.6
77.9
6.84
7.58
11.2
3 keV
Cross Section
(× 10−9) (cm2)
Total
6.33
LBH
4.37
Non‐LBH
1.96
Total
11.0
LBH
9.12
Non‐LBH
1.87
36.0
535.0/110b
37.5
132.1
11.3
16.4
27.4
5 keV
Cross Section
(× 10−9) (cm2)
Total
6.67
LBH
4.60
Non‐LBH
2.07
Total
13.8
LBH
11.4
Non‐LBH
2.34
50.2
537/120b
39.6
182.0
13.6
22.1
38.5
7 keV
Cross Section
(× 10−9) (cm2)
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AJELLO ET AL.: UV EMISSIONS FROM PROTON IMPACT ON N2
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1375.2
1390.4
1400.8
1420.0
1438.4
1454.4
14
15
16
17
18
19
1332.0
12
1344.0
1314.4
11
13
1306.4
1464.0
1443.2
1429.6
1411.2
1396.0
1383.2
1353.6
1337.6
1325.6
1310.4
Wave‐length Wave‐length
Start (Å)
Peak (Å)
10
Feature Number
Table 1. (continued)
1427
1430
1441
1444
1450
1459
1464
1353
1354
1382
1384
1395.9
1398
1411.9
1411.9
1415.9
1339.0
1342.01‐
1343.49
1325.3
1326.3
1326.564‐
1327.924
1309.22
1310.18
1310.540
1310.950
1312
Wave‐length
Peak Theory/
Observed (Å)
1469.6
1454.4
1435.2
1420.0
1400.8
1390.4
1360.0
1344.0
1332.0
1314.4
Wave‐length
End (Å)
a(5,3)
a(2,1)
a(6,4)
a(3,2)
a(0,0)
a(4,3)
a(1,1)
a(6,2)
a(3,0)
a(5,2)
a(2,0)
a(6,3)
a(3,1)
NI(2Po ← 2D)
a(4,2)
a(1,0)
a(5,1)
NI,II
a(4,0)
b(1,12)
NI(2Po ← 2P)
NI(2Po ← 2P)
b′(1,12)
c′4(0,13)
NI(2Po ← 2D)
NI(2Po ← 2D)
a(6,1)
Wavelength
Identificationa
8.56
12.9
9.27
Total
17.0
11.6
LBH
Non‐LBH
5.36
12.6
Total
10.6
7.25
LBH
Non‐LBH
3.35
7.41
5.92
5.31
20.2
Total
5.61
LBH
1.74
Non‐LBH
3.87
Total
19.6
LBH
16.7
Non‐LBH
2.94
Total
4.74
LBH
1.57
Non‐LBH
3.18
18.5
2 keV
Cross Section
(× 10−9) (cm2)
4.75
11.1
Total
3.11
LBH
0.96
Non‐LBH
2.15
Total
9.79
LBH
8.32
Non‐LBH
1.47
Total
4.60
LBH
1.52
Non‐LBH
3.08
11.9
1 keV
Cross Section
(× 10−9) (cm2)
15.0
11.2
Total
19.5
LBH
13.4
Non‐LBH
6.18
14.3
5.43
24.3
Total
6.86
LBH
2.13
Non‐LBH
4.73
Total
24.0
LBH
20.4
Non‐LBH
3.60
Total
6.10
LBH
2.01
Non‐LBH
4.09
23.6
3 keV
Cross Section
(× 10−9) (cm2)
15.7
9.80
Total
23.7
LBH
16.2
Non‐LBH
7.48
14.0
6.91
22.6
Total
11.3
LBH
3.51
Non‐LBH
7.82
Total
28.0
LBH
23.8
Non‐LBH
4.19
Total
5.73
LBH
1.89
Non‐LBH
3.83
23.8
5 keV
Cross Section
(× 10−9) (cm2)
14.4
9.85
Total
25.9
LBH
17.7
Non‐LBH
8.19
14.0
6.52
23.4
Total
15.0
LBH
4.65
Non‐LBH
10.3
Total
32.0
LBH
27.2
Non‐LBH
4.79
Total
6.56
LBH
2.17
Non‐LBH
4.40
23.6
7 keV
Cross Section
(× 10−9) (cm2)
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1566.4
25
1587.2
1553.6
1529.6
1506.4
1492.8
1473.6
1508.1
1515.3
1523
1530
1555
1560
1570
1576
1585
1592
1474
1479
1489
1493
1493.3
1501
Wave‐length
Peak Theory/
Observed (Å)
19.6
8.67
10.2
Total
27.2
LBH
2.83
Non‐LBH
24.4
14.5
5.16
2 keV
Cross Section
(× 10−9) (cm2)
23.7
10.7
11.6
Total
40.0
LBH
4.16
Non‐LBH
35.9
17.1
6.03
3 keV
Cross Section
(× 10−9) (cm2)
22.0
8..59
10.1
Total
66.5
LBH
6.92
Non‐LBH
59.6
17.9
7.07
5 keV
Cross Section
(× 10−9) (cm2)
23.4
9.0
10.6
Total
91.6
LBH
9.53
Non‐LBH
82.1
19.5
8.80
7 keV
Cross Section
(× 10−9) (cm2)
2.60 × 10−17 cm2 5.77 × 10−17 cm2 7.13 × 10−17 cm2 8.82 × 10−17 cm2 9.98 × 10−17 cm2
1100–1600
1.32 × 10−17 cm2 1.58 × 10−17 cm2 1.84 × 10−17 cm2 1.82 × 10−17 cm2 1.90 × 10−17 cm2
18.7
8.92
7.39
Total
11.0
LBH
1.15
Non‐LBH
9.88
10.9
5.13
1 keV
Cross Section
(× 10−9) (cm2)
2.30 × 10−17 cm2 4.21 × 10−17 cm2 4.90 × 10−17 cm2 4.83 × 10−17 cm2 5.05 × 10−17 cm2
a(4,4)
a(1,2)
a(5,5)
a(2,3)
a(0,2)
a(4,5)
a(1,3)
a(5,6)
a(2,4)
a(6,7)
a(5,4)
a(2,2)
a(6,5)
a(3,3)
NI(2Do ← 2P)
a(0,1)
Wavelength
Identificationa
1262–2500
1262–1600
1599.2
1566.4
1536.0
1520.8
1500.8
1482.4
Wave‐length
End (Å)
NIST atomic wavelength tables, Ajello and Shemansky [1985], and J. M. Ajello (private communication, 2010).
H Ly a cascade cross section from Van Zyl and Neumann [1988].
b
a
Totals LBH emission
cross section (37.6%
of band system)
Totals LBH emission
cross section (full
band system)
Total non‐LBH
emission cross section
1545.6
24
1500.8
22
1520.8
1485.6
21
23
1469.6
Wave‐length Wave‐length
Start (Å)
Peak (Å)
20
Feature Number
Table 1. (continued)
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AJELLO ET AL.: UV EMISSIONS FROM PROTON IMPACT ON N2
the optic axis in our experiment [Van Zyl and Gealy, 1987].
We have calculated the escape loss of cascade‐producing 3s,
4s and 5s atoms from our FOV for 1–7 keV. For 3s atoms
moving perpendicular to the optic axis of the spectrometer
we find that for the 3s state the maximum percentage of
Balmer‐a cascade observed occurs at 1 keV with 25% at
1 keV and dropping to 10% at 7 keV. For the cascade
production by 4s and 5s states the percentage observed
drops to less than 10% at all energies. In contrast, only a
small amount of the nd→2p→1s Balmer series will escape
for the nd series (n ≥ 3). The nd states have a shorter lifetime
of about a factor of 10 for the same n value (e.g., 15 ns for
the H(3d) state). We find that all the cascade emission from
3d (Ha) is observed at 1 keV and nearly 65% of the cascade
emission from 5d (Hg) at 1 keV. Dawson and Loyd [1977]
and Loyd and Dawson [1975] have shown the ratio of
excitation cross sections for electron capture (cross‐section
ratio of 3s/3d states) described by equation (3) to vary from
0.8 to 1.4 (near unity to a first approximation) over the
proton energy range of 3.2–7 keV. Outside of the range of
1–10 keV Hughes et al. [1970] have found the 3s,3p,3d
excitation cross‐section ratios from 5 to 200 keV to vary
over a much larger range. The nd series members for 3 ≤ n ≤ 6
contribute approximately half the total cascade cross‐section
H Ly a as determined by Van Zyl and Neumann [1980,
1988] and are mainly observable in this experiment [Van Zyl
et al., 1989]. We correct in approximate fashion for the drift
loss of ns and nd atoms by assuming all the nd atoms
are observed and all the ns atoms are lost. For example the
H(3d‐2p), cascade transition has an e‐folding distance to
radiate Balmer‐a, of less than 3 cm for fast H atoms of the
range of 1–7 keV.
[27] The feature identifications are partly based on high‐
resolution FUV spectra from e− + N2 collisions in the
wavelength range of 1100–1350 Å (J. M. Ajello, private
communication, 2010) and partly based on AS85 for the
wavelength range of 1350–1600 Å. The relative intensities
of the LBH bands (v′, v″) are measured to be proportional to
Franck‐Condon factors given by AS85. We observe 57% of
the LBH band system from 1262 Å to 1600 Å. The proton
emission cross section of the remaining portion of the band
system is estimated as by AS85 using Franck‐Condon factors. The strongest features for each progression are feature
24, the (0,2) band (blended with the 4,5 band) at 1555 Å for
the v′ = 0 progression, feature 19, the (1,1) band (blended
with the (5,4) band) at 1464 Å for the v′ = 1 progression,
feature 13, the (2,0) band (blended with the (6,2) band) at
1356 Å for the v′ = 2 progression, feature 14 the (3,0) band
(blended with the (5,2) band) at 1383 Å for the v′ = 3
progression and feature 11, the (4,0) band (blended with
some N I multiplets and a b(1,12) band) at 1325 Å for the
v′ = 4 progression. The peak proton‐impact‐induced emission cross section of the LBH band system (1260–2500 Å)
was measured in this experiment over the energy range
of 1–7 keV to be 5.05 ± 1.52 × 10−17 cm2 at 7 keV. We
compare this proton‐impact‐induced emission cross section
to the peak electron‐impact‐induced emission cross section
of the LBH band system. The electron‐impact‐induced
emission cross section of the LBH band system is 6.3 ±
1.1 × 10−18 cm2 at 100 eV given by Young et al. [2010] is
used to normalize cross sections of other energies. The peak
electron‐impact‐induced emission cross section, based on
the measured relative electron excitation function by Young
et al. [2010], occurs at 20 eV with an extrapolated value of
1.74 ± 0.30 × 10−17 cm2. A comparison of proton and
electron‐impact‐induced emission cross sections indicates a
greater efficiency for LBH emission production from protons than electrons at the respective peak particle energies
by at least a factor of about three.
[28] Direct excitation of the LBH band system results
from the transition X 1S+g → a 1Pg. The overlapping of
the nearby a′ X 1S+g and w 1Du vibronic levels, are coupled
to the a 1Pg state via exceptionally weak dipole‐allowed
infrared transitions, and can produce significant but slow
cascade either radiatively or by collision‐induced electronic
transitions (CIET) [Eastes and Dentamaro, 1996; Eastes,
2000a, 2000b]. The lifetime of the slow cascade states are
greater than 1 ms and slow cascade is not observed in this
experiment, since collisional deactivation of these long‐
lived metastables occurs at the walls. Additionally, the fast
cascade to the a 1Pg state results primarily from the c′4 1S+u
state [Shemansky et al., 1995; Filippelli et al., 1984] but its
contribution has been shown to be less than 1%. Thus, neither
fast nor slow cascade makes important contributions to the
measured LBH proton‐impact‐induced emission cross sections (<5%). We have previously estimated cascade contribution to the electron impact emission cross section for slow
cascade from a′ and w states to be significant with a ratio of
slow cascade to direct excitation to be ∼0.3 at 20 eV [Ajello
et al., 2010; Young et al., 2010].
3.2. H+2 and H+3 Emission Cross Sections
[29] To our knowledge, the UV emission cross sections
from collisions of H+2 and H+3 molecular ions with N2 have
never been studied before. A recent study in the visible
of the Balmer series members Hb and Hg over the energy
range of 3–10 keV has been performed by Lee and Lin
[2002] using a similar Colutron source. For the H+2 ion
beam the collision processes with N2 are described by
the following:
þ
1
þ
Hþ
2 ð17 keVÞ þ N2 ! H2 þ N2 * a Pg ! H2 þ N2 þ hLBH ;
ð4Þ
þ
1
Hþ
2 ð17 keVÞ þ N2 ! H þ H þ N2 * a Pg
þ
! H þ H þ N2 þ hLBH ;
ð5Þ
þ
Hþ
2 ð17 keVÞ þ N2 ! H2 þ N2 *ðrepulsiveÞ
þ
! H þ H þ N þ N þ h120;149:3 nm;...: ;
ð6Þ
þ
Hþ
2 ð17 keVÞ þ N2 ! H þ H þ N2 *ðrepulsiveÞ
þ
! H þ H þ N þ N þ h120;149:3 nm;...: ;
ð7Þ
o*
o
þ
þ H þ Nþ
Hþ
2 ð17 keVÞ þ N2 ! H
2 ! H þ H þ N2 þ hLy ;
ð8Þ
o*
þ Hþ þ N2 ! Ho þ Hþ þ N2 þ hLy ;
Hþ
2 ð17 keVÞ þ N2 ! H
ð9Þ
where in equations (8) and (9) the N+2 neutral or N2 ion may
further dissociate.
[30] The emission cross sections of the process H+2 and
H+3 + N2 at 5 keV ion energy are listed in Table 2 along
14 of 21
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AJELLO ET AL.: UV EMISSIONS FROM PROTON IMPACT ON N2
with a comparison to the H+ emission cross section also
at 5 keV from Table 1. The emission spectrum is shown
in Figure 7. The only previous experiment on H+2 ‐N2 was
performed by Van Zyl et al. [1964, 1967] who measured
the H Ly a emission cross section for H+2 ‐N2 collisions
using an ion drift‐tube technique and found values nearly
identical to those in Table 2 for H+ and H+2 .
[31] It is interesting to compare the H Ly a cross sections for the three projectiles and note that the emission
cross sections (sem) at 5 keV vary in the order sem(H+3 ) >
sem(H+2 ) > sem(H+) in the same order as found by Lee and
Lin [2002] for Ha and Hb. We find the H Ly a emission
cross‐section ratios, for H+3 , H+2 , H+ ions, to be 1.50:1.46:1,
respectively, compared to the same order of 1.52 > 1.38 > 1
for Hb (4861 Å) [Lee and Lin, 2002]. On the other hand
we find the LBH cross sections to be largest for the H+2
projectile ion. Lee and Lin have done a thorough job in
explaining a theoretical understanding of the intermediate
molecular states involved in the collisional excitation and
charge transfer processes responsible for the emissions. We
have not yet studied other energies besides our reported
5 keV energy.
3.3. Emission Cross Sections of N I Multiplets From
1 to 130 keV
[32] We show in Figures 8 a semilog plot of the previously published work of collisions of H+ with N2 as
reviewed by Avakyan et al. [1998] along with the results
from this paper. The uncertainty of these values is about
20% for energies between 10 and 100 keV. The uncertainty
of our work is the root‐sum‐square uncertainty of (1) 10%
in the relative FUV calibration, (2) 20% uncertainty of
the absolute calibration of H Ly a, (3) 5% uncertainty in the
H cascade model contribution to drift loss outside of the
field‐of‐view from long‐lived ns and nd states, (4) 10%
uncertainty in the LBH swarm model accounting for drift
loss of excited a 1Pg molecules outside of the field‐of‐view
prior to radiating from the use of the 55ms lifetime, (5) 12%
uncertainty in the use of a cross beam approximation to a
swarm experiment model, (6) 5% uncertainty in Faraday
cup current loss to grounded shield, (7) 3% uncertainty to
pressure drift, and (8) 10% uncertainty in measured areas
from blended feature overlap and S/N statistical uncertainty.
The root sum square uncertainty of our reported LBH and
N I emission cross sections is 30%.
[33] The only work performed previously on the N I
multiplets was from 10–130 keV. This work was performed
by Dahlberg et al. [1967]. In their review, Avakyan et al.
[1998] renormalized the published data by newer cross
sections for H Ly a from H+ + N2 collisions. The proton‐
excited H Ly a emission cross section has been measured
over the energy range from 120 eV–120 keV by the combined work of Van Zyl and Neumann [1988] and Dahlberg
et al. [1967]. Our results are the first for proton energies
below 10 keV for the N I multiplets. This region (1–10 keV)
contains the peak value of the LBH and H Ly a emission
cross section whereas the N I multiplets have peak cross
sections in the 25–50 keV energy range. We now have for
the first time a complete set of proton impact cross sections
for the energy range of 1–130 keV for the N I multiplets. We
plan to extend the energy range of LBH emission cross
A00K03
sections in similar fashion to higher and lower energies to
access both the threshold and Born energy regions.
4. The Auroral Application
[34] Proton auroral models make use of three classes of
cross sections, namely proton impact, H impact, and electron impact (the latter to address secondary electron impact
excitation and ionization). A full set of primary (proton
and H) impact cross sections for N2, O2, and O is needed to
describe the degradation with altitude of these primary
particles. The result is altitude profiles of particle fluxes
versus energy (and pitch angle unless integrated over this
variable) for proton, H, and secondary electrons. For models
solving linear transport equations for these three particles
[Basu et al., 1993; Strickland et al., 1993; Galand and
Lummerzheim, 2004] (the latter joining together the models of Galand et al. [1997] and Lummerzheim and Lilensten
[1994]), volume excitation and ionization rates follow from
integrations of the product of appropriate cross sections with
these fluxes. Monte Carlo models [e.g., Gérard et al., 2000,
2001; Solomon, 2001; Fang et al., 2004] obtain these rates
by scoring events during the random walk process. It is thus
clear that our new measurements, specific to a given FUV
emission feature, help quantify part, but not all emissions.
The secondary electron component, in general, does not lack
the needed cross section for its quantification. The H atom
component, on the other hand, is usually the most uncertain
among the three components due to a general lack of laboratory cross‐section measurements that are more difficult
to perform than those for proton impact. This is the case for
LBH that led to the need for some type of cross‐section
specification in the work reported by Strickland et al.
[1993]. As reported in that paper, an estimate was provided by B. Van Zyl (private communication, 1989).
Another key feature lacking H impact measurements is OI
1356 Å (impact on O being more important than on O2).
In this case, proton impact on O is unimportant due to the
transition being spin‐forbidden. While we have no current
plans for H impact measurements, plans are being formulated for extending current work to energies into the Born
region for proton impact on N2, O2 and O.
[35] For remote sensing applications, the value of the new
cross sections being reported is twofold: first to compare
with existing representations in models (e.g., with the proton
impact LBH cross section reported by Strickland et al.
[1993]) and second to provide cross sections previously
unspecified. Key features that this pertains to are NI 1200 Å
and NI 1493 Å that may comprise portions of instrument
bands containing Ly a and LBH. Such bands can be found
on SSUSI and GUVI in their imaging modes and in the case
of LBH, applies to the band designated as LBHS (S is for
short) whose band limits are ∼1400 Å and ∼1530 Å. Still
lacking are H impact cross sections for these features that, as
a first approximation, can be set equal to their proton
counterparts. Another approach is to allow the relative
strength between H and proton cross sections to reflect that
of a feature where both components have previously been
specified (e.g., for LBH as discussed above). In the near
future, simple scaling techniques such as these can be replaced
with anticipated H impact cross sections from extension of
our current work.
15 of 21
1205.6
1223.2
1236.8
1266.4
1288.8
5
6
7
8
9
1183.2
3
1190.4
1172.8
2
4
1159.2
1
Feature Number
Wave‐length
Start (Å)
16 of 21
1297.6
1272.8
1243.2
1215.2
1224.0
1199.2
1188.8
1176.0
1168.0
Wave‐length
Peak (Å)
1291.8
1298.25
1176.51
1176.53
1177.695
1188.971
1189.249
1190.494
‐
1190.019
1199.89
1200.45
1201.85
1215.7
1225.026
1225.368
1225.374
1228.407
1228.785
1228.791
1243.171
‐
1243.313
1273.2
1275.038
1276.19
1277.41
1159–1172
Wave‐length
Peak Theory/
Observed (Å)
1306.4
1280.0
1252.8
1223.2
1236.8
1205.6
1190.4
1183.2
1172.8
Wavelength
End (Å)
b(1,11)
a(5,0)
a(6,0)
NII(3Do ← 3P)
b′(1,12)
c′4(0,12)
NI(g 4So ← 4P)
NI(g 4So ← 4P)
NI(g 4So ← 4P)
H Ly a
NI(2Po ← 2D)
NI(2Po ← 2D)
NI(2Po ← 2D)
NI(2Po ← 2P)
NI(2Po ← 2P)
NI(2Po ← 2P)
NI(2Do ← 2D)
NI(2Do ← 2D
NI(2Po ← 2,4P)
NI(2Do ← 4P)
NI(2Do ← 2P)
NI(2Do ← 4F)
NI(2Do ← 4P, 2F)
NI(2Do ← 2P)
NI(2Do ← 2P
NI(2Do ← 2P
NI(g 4So ← 4P)
NI(g 4So ← 4P)
NI(2Po ← 2,4P, 2,4F)
Wavelength
Identificationa
Total
14.9
LBH
10.3
Non‐LBH
4.63
Total
15.9
LBH
13.2
55.8
36.0
Total
6.33
LBH
4.37
Non‐LBH
1.96
Total
11.0
LBH
9.12
782
61.6
535.0/110b
37.5
18.3
11.3
181.6
25.5
16.4
132.1
37.6
H+2 5 keV
Cross Section
(× 10−19) (cm2)
27.4
H+ 5 keV
Cross Section
(× 10−19) (cm2)
Table 2. The H+, H+2 , and H+3 Impact Induced Fluorescence Spectral Identifications and Emission Cross Sections of N2 From 1150 to 1600 Å
Total
5.07
LBH
3.50
Non‐LBH
1.57
Total
8.69
LBH
7.21
35.5
804
51.7
214.7
19.7
28.0
45.4
H+3 5 keV
Cross Section
(× 10−19) (cm2)
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AJELLO ET AL.: UV EMISSIONS FROM PROTON IMPACT ON N2
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1332.0
1344.0
1375.2
1390.4
1400.8
1420.0
1438.4
13
14
15
16
17
18
1314.4
11
12
1306.4
Wave‐length
Start (Å)
10
Feature Number
Table 2. (continued)
17 of 21
1443.2
1429.6
1411.2
1396.0
1383.2
1353.6
1337.6
1325.6
1310.4
Wave‐length
Peak (Å)
1427
1430
1441
1444
1450
1353
1354
1382
1384
1395.9
1398
1411.9
1411.9
1415.9
1339.0
1342.01‐
1343.49
1325.3
1326.3
1326.564‐
1327.924
1310.540
1310.950
1312
Wave‐length
Peak Theory/
Observed (Å)
1454.4
1435.2
1420.0
1400.8
1390.4
1360.0
1344.0
1332.0
1314.4
Wavelength
End (Å)
2
a(5,3)
a(2,1)
a(6,4)
a(3,2)
a(0,0)
a(6,2)
a(3,0)
a(5,2)
a(2,0)
a(6,3)
a(3,1)
NI(2Po ← 2D)
a(4,2)
a(1,0)
a(5,1)
NI,II
a(4,0)
b(1,12)
2 o
NI( P ← 2P)
NI(2Po ← 2P)
2 o
NI( P ← D)
NI(2Po ← 2D)
a(6,1)
Wavelength
Identificationa
11.6
Total
24.6
LBH
16.8
Non‐LBH
7.77
13.7
Total
23.7
LBH
16.2
Non‐LBH
7.48
14.0
9.80
8.99
6.91
19.5
Non‐LBH
2.70
Total
13.5
LBH
4.19
Non‐LBH
9.32
Total
25.4
LBH
21.6
Non‐LBH
3.81
Total
9.04
LBH
2.98
Non‐LBH
6.06
20.5
Non‐LBH
1.87
Total
11.3
LBH
3.51
Non‐LBH
7.82
Total
28.0
LBH
23.8
Non‐LBH
4.19
Total
5.73
LBH
1.89
Non‐LBH
3.83
23.8
22.6
H+2 5 keV
Cross Section
(× 10−19) (cm2)
H+ 5 keV
Cross Section
(× 10−19) (cm2)
4.95
Total
12.2
LBH
3.85
Non‐LBH
8.33
5.84
3.46
8.87
Non‐LBH
1.48
Total
9.31
LBH
2.89
Non‐LBH
6.43
Total
16.5
LBH
14.1
Non‐LBH
2.48
Total
2.58
LBH
0.85
Non‐LBH
1.73
9.91
H+3 5 keV
Cross Section
(× 10−19) (cm2)
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AJELLO ET AL.: UV EMISSIONS FROM PROTON IMPACT ON N2
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18 of 21
1469.6
1485.6
1500.8
1520.8
1545.6
1566.4
20
21
22
23
24
25
1587.2
1553.6
1529.6
1506.4
1492.8
1473.6
1464.0
Wave‐length
Peak (Å)
1508.1
1515.3
1523
1530
1555
1560
1570
1576
1585
1592
1459
1464
1474
1479
1489
1493
1493.3
1501
Wave‐length
Peak Theory/
Observed (Å)
5.74 × 10−17 cm2
12.0 × 10−17 cm2
4.83 × 10−17 cm2
8.82 × 10−17 cm2
1262–2500
1100–1600
1262–1600
2.16 × 10−17 cm2
15.1
12.6
Total
72.7
LBH
7.56
Non‐LBH
65.1
18.8
9.48
15.3
H+2 5 keV
Cross Section
(× 10−19) (cm2)
1.82 × 10−17 cm2
8.59
10.1
Total
66.5
LBH
6.92
Non‐LBH
59.6
17.9
7.07
15.7
H+ 5 keV
Cross Section
(× 10−19) (cm2)
30.8
a(4,4)
a(1,2)
a(5,5)
a(2,3)
a(0,2)
a(4,5)
a(1,3)
a(5,6)
a(2,4)
a(6,7)
a(4,3)
a(1,1)
a(5,4)
a(2,2)
a(6,5)
a(3,3)
NI(2Do ← 2P)
a(0,1)
Wavelength
Identificationa
22.0
1599.2
1566.4
1536.0
1520.8
1500.8
1482.4
1469.6
Wavelength
End (Å)
NIST atomic wavelength table, Ajello and Shemansky [1985], and J. M. Ajello (private communication, 2010).
a
Totals LBH emission
cross section (37.6%
of band system)
Totals LBH emission
cross section (full
band system)
Total non‐LBH
emission cross section
1454.4
Wave‐length
Start (Å)
19
Feature Number
Table 2. (continued)
12.7 10−17 cm2
2.53 × 10−17 cm2
0.95 10−17 cm2
13.4
5.80
5.45
Total
58.2
LBH
6.05
Non‐LBH
52.2
8.82
4.30
8.14
H+3 5 keV
Cross Section
(× 10−19) (cm2)
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AJELLO ET AL.: UV EMISSIONS FROM PROTON IMPACT ON N2
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AJELLO ET AL.: UV EMISSIONS FROM PROTON IMPACT ON N2
[36] There is still the issue of cross sections in transport
models extending to 100s of keV whereas our current
measurements stop at 7 keV. Nevertheless, it is still useful
to know how well the cross sections in models agree at
these lower energies. For LBH, our measured cross section
exceeds that used by Strickland et al. and Knight et al.
below 7 keV but merges at this current upper limit of
observation. Plans (D.J.S.) are to adopt the new measured
values that, while increasing calculated emission, will do so
only modestly given that the bulk of emission arises from
proton/H fluxes above 7 keV.
[37] The work started by Knight et al. has the potential
to complement our ongoing laboratory work. The data
addressed in that paper come from coincident observations
by SSUSI (auroral FUV output) and SSJ/5 (auroral particle
input). For electron and proton precipitation, monoenergetic
yields were introduced for Ly a (for proton precipitation),
LBHS, and LBHL that provide simulated SSUSI observations when integrated with SSJ/5 measurements (the
integrand is the product of a yield and a particle spectrum).
Such yields for proton precipitation, contain the three emission components discussed above. While making coincident
measurements from the same observing platform allows
for detailed statistical studies, one drawback specific to the
current design of the particle sensor is proton measurements
limited to energies at and below 30 keV (not an issue for
electron measurements that have the same range). This
requires extrapolation as discussed in some detail by Knight
et al. From a statistical perspective, any bias in extrapolations
introduces an error in median values of the ratio of column
emission rates from SSUSI and from the use of monoenergetic yields with SSJ/5 data. Were there no biases, either
from extrapolations or calibrations, the median ratio for each
feature would be unity if the correct cross sections were used
to calculate the yields (for either electron or proton aurora).
Deviations from unity then point out errors in magnitudes and
energy dependence of assumed cross sections. The primary
conclusion of Knight et al. is that larger proton and H LBH
cross sections are needed based on the above discussed
median ratio for proton aurora having values in excess of
two for LBHS and LBHL. As noted in section 1, this provided the motivation for the work started by us to directly
measure cross sections of interest to proton aurora. Since the
publication of the Knight paper, Knight and colleagues have
performed detailed testing of the extrapolation algorithm
using TED/MEPED data (all NOAA 17 data from 2003 and
2004) and have uncovered a bias whose removal will
decrease the LBHS and LBHL median ratios. Details given
by Correira et al. (submitted manuscript, 2010) are currently
under review. The observed bias is not enough to reduce
the ratios to unity and thus still calls for increases in cross
sections assuming small relative calibration errors between
SSUSI and SSJ/5 (say, <20%).
5. Summary
[38] Energetic electron, proton/hydrogen, and secondary
electron interactions with ambient neutral species (N2, O or
O2) make them the primary source of auroral emissions.
Coupling, Energetic and Dynamics of Atmospheric Regions
(CEDAR)/Phase III recently stated that proton studies offer
an excellent means for investigating magnetospheric storms
A00K03
and magnetosphere‐ionosphere coupling processes. Emissions from electron and proton aurora have been and continue
to be observed from satellites (e.g., the Global Ultraviolet
Imager (GUVI) aboard the NASA Thermosphere Ionosphere
Mesosphere Energy and Dynamics (TIMED) and the Special
Sensor Ultraviolet Spectrographic Imager (SSUSI) onboard
Defense Meteorological Satellite Program (DMSP) satellites
and from ground‐based observations.
[39] Recently the Image satellite using its FUV imaging
system provided a global view of the north aurora from
both proton and electron precipitation [Coumans et al.,
2002, Figures 1 and 11]. The emission models of auroral
brightness use a proton transport code that makes use of
the in situ measurements from particle measurements on
board the NOAA‐TIROS satellites. For example, the NOAA
15 and 16 orbital tracks provide in situ measurements of
the proton distribution function over the energy range from
50 eV to 800 keV and the electron distribution function
over the energy range from 50 eV to 1000 keV. The peak of
the proton energy distribution function is found to be in the
energy range 10–15 keV, a range that is coincident with
large values of the laboratory‐measured N2 emission cross
sections shown in Figure 8. With the ability to be able to
discriminate the precipitating particles it has now become
possible to analyze spacecraft global images in order to
correctly and separately assess ionosphere conductivity,
heating, and composition changes. To improve the values of
these models we have recently produced in the laboratory
accurate electron impact [Young et al., 2010] and proton
impact N2 emission cross sections.
[40] A review of emission cross sections for proton impact
available to aeronomers by Avakyan et al. [1998] shows a
dearth of data; the cross sections are either not reliable or
undetermined. We have begun to make a major improvement
in the laboratory cross‐section database by providing for
the first time experimental proton‐excited N2 FUV (1100–
1600 Å) emission cross sections for the LBH band system
and the UV resonance multiplets at 1200, 1243, 1492 and
1743 Å over the proton energy range of 1–7 keV. We
have established benchmark LBH emission cross sections
near peak energy at ∼5 keV for use with proton energy loss
models in the terrestrial atmosphere. Improved specification
of auroral types and prediction capabilities for (Mesosphere,
Lower Thermosphere/Ionosphere) MLTI densities will result
from this project. The determination of the physical MLTI
region parameters of the Earth’s thermosphere is only as
accurate as the cross sections used in the analysis.
[41] We have used the recommended H Ly a cross sections of Avakyan et al. [1998], who depend upon the
accuracy of the combined work by Dahlberg et al. [1967],
Van Zyl and Neumann [1988], and Birely and McNeal
[1971] to normalize the emission cross sections of N2 in
this study. In future, we plan to extend the energy range
for the proton emission cross sections into the Born region
well beyond 10 keV and into the threshold region below
1 keV.
[42] Acknowledgments. This work was performed at the Jet Propulsion Laboratory (JPL), California Institute of Technology (Caltech), under
a contract with the National Aeronautics and Space Administration
(NASA). We gratefully acknowledge financial support through NASA’s
Geospace and Planetary Atmospheres programs, the National Science
19 of 21
A00K03
AJELLO ET AL.: UV EMISSIONS FROM PROTON IMPACT ON N2
Foundation Office under grant 0850396, the Astronomy and Physics
Research and Analysis Program, the Cassini Data Analysis Program
and Space Physics Program Offices. We thank Lars Wahlin for helping
establish operation of the Colutron ion source at JPL and Ronald
Cummings for his technician support. We have benefited from discussions
with B. Van Zyl of the University of Denver.
[43] Philippa Browning thanks Anil Bhardwaj and another reviewer for
their assistance in evaluating this paper.
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