Flux transfer events:
Looking ahead to MMS
(Evolution and current status)
GEM Tutorial
Robert Fear & Lorenzo Trenchi
Space Environment Physics group
University of Southampton, UK
Outline
• Early work (1978-2000)
• Post 2000
– Cluster
– Double Star
– THEMIS
– MESSENGER
• A look ahead to MMS
Solar wind flow
Sun
Magnetic flux is “opened”
“Open” flux is “closed”
Closed
Interplanetary Magnetic Field [IMF]
Open
Closed
Dungey’s (1961)
Open Magnetosphere
Fast flows at Alfvén speed:
Evidence for magnetopause reconnection
Acceleration
• First direct observation of predicted flows made at magnetopause by
Paschmann et al., Nature, (1979)
• Direct evidence for driving of Earth’s magnetosphere
by dayside reconnection
11 months earlier…
Magnetosheath
Magnetosphere
L
M
N
•
•
•
•
•
Russell & Elphic (1978, 1979) introduced boundary normal
coordinate system
Identified bipolar signatures normal to MP
Enhancement in |B|
Interpreted as signature of localised flux tube: ‘flux transfer events’
NB “Flux erosion events” reported by
Haerendel et al. (1978) shown to be the same
(Rijnbeek & Cowley, Nature, 1984)
FTE polarity
Fear et al. (2008)
L
Northward motion
(standard polarity):
BN
Russell & Elphic
(1978)
Magnetosphere
Magnetosheath
M
After Russell & Elphic
(1978)
L
Southward motion
(reverse polarity):
BN
N
Rijnbeek et al.,
Nature (1982)
FTE polarity
Berchem & Russell (1984)
• Standard polarity signatures observed predominantly in
northern hemisphere and reverse polarity signatures in
southern hemisphere
• Evidence for subsolar reconnection even for strong IMF BY
(i.e. component reconnection)
Observed throughout the Solar System…
•
Similar features seen at:
Earth
Jupiter
(Walker & Russell, 1985)
Mercury
(Russell & Walker, 1985;
Slavin et al., 2009, 2010, 2012;
Imber et al., 2014)
•
Interestingly not clearly seen at Saturn (e.g. Lai et al., 2012), despite
magnetopause observations of reconnection (McAndrews et al., 2008) and
auroral indications that it may be bursty (Badman et al., 2013)
•
Suggested may be occurring at the heliopause? (Schwadron & McComas, “Is
Voyager 1 inside an interstellar flux transfer event?”, Ap. J., 2013)
Dorfman et al., GRL, (2013)
…and in the lab
Cusp signatures of FTEs
• Steady reconnection leads to
dispersion pattern in cusp
precipitation (velocity filter
effect)
• Pulses in reconnection
(FTEs) cause discrete ‘steps’
in cusp precipitation
Lockwood et al. (1993)
See also
Newell & Meng (1991);
Escoubet et al. (1992);
Onsager et al. (1995);
Trattner et al. (2015)
Auroral/radar signatures of FTEs
Fasel (1995)
See also: Sandholt et al. (1986); Elphic & Lockwood (1990);
Pinnock et al. (1993); Neudegg et al. (2000); McWilliams et al.
(2000); Wild et al., (2001, 2003, 2005a,b, 2007)
Provan et al. (1998)
IMF dependence
Kuo et al. (1995)
• FTE signatures at dayside magnetopause
occur predominantly for southward (or
BY-dominated) IMF
– Consistent with signature of bursty
reconnection
• First statistical studies to extend postterminator revealed presence of FTEs on
low latitude flank for strongly northward
IMF (Kawano & Russell, 1997a,b)
– Expect reconnection for northward
IMF at high latitudes, but FTEs
observed equatorial
– Cluster observations show that
equatorward motion is observed –
consistent with balance of forces at
high latitude reconnection site
(Fear et al., 2005), or suppression of
signatures on day side for BZ > 0
(Sibeck, 2009)?
Kawano & Russell (1997)
Milan et al. (2000)
Auroral/radar signatures of FTEs
Provan et al. (1998)
Alternative models of FTEs
ZGSM
YGSM
N
Magnetosheath
Magnetosphere
L
Lee & Fu (1985)
Liu & Hu (1988)
Southwood et al. (1988) &
Scholer (1988a)
• For interesting background on
development of these models,
see intro to Farrugia et al. (2011)
Why does this matter?
Lockwood & Wild (1993)
• Estimates of flux transferred by
individual FTEs differ wildly
• In situ estimates typically range from 0.11% of total open flux being opened in
single burst at Earth (e.g. Saunders et al.,
1984; Hasegawa et al., 2006)
– Distribution of inter-FTE times
(spectrum of reconnection rates), but
mean ~ 8 mins
– ⇒ 0.8-8% of polar cap refreshed/hour
[⇒FTEs secondary to steady state?]
• Ionospheric estimates indicate that up to
10% can be opened in one go (Milan et
al., 2000)
– ⇒ FTEs dominant?
Milan et al. (2000)
Why does this matter?
Imber et al. (2014)
• NB MESSENGER observations show role of FTEs at Mercury
proportionally much more significant (e.g. 5% [Slavin et al., 2010], 2%
[Imber et al., 2014])
• Coupled with much faster repetition rate (~2s), indicates that FTEs at
Mercury are likely to be the dominant driver of Mercury’s
magnetospheric dynamics
(Imber et al., 2014)
Why does this matter?
Fear et al. (2008)
• Difference may be due to assumptions about structure
(see Fear et al., 2008)
Distinguishing between mechanisms
• Lockwood & Hapgood (1998)*
modelled the ion population
crossing an open magnetopause
as a function of time since
reconnection
• Found observed ion spectrum
evolved continuously, consistent
with single X-line model
• On balance, favoured single Xline mechanism over
Russell/Elphic or multiple Xlines for this event
*“On the cause of a
magnetospheric flux transfer
event”, JGR
Converging jets
Hasegawa et al. (2010)
•
Converging jets
indicative of
multiple X-lines
reported
(Hasegawa et al.,
2010; Trenchi et
al., 2011; Øieroset
et al., 2011)
•
Such “in vivo” events are
rarely observed (~1% of
events observed by
Zhang et al., 2012)
Grad Shafranov reconstruction
• Development of analysis
techniques such as Grad
Shafranov reconstruction
(Sonnerup et al., 2004, 2006;
Hasegawa et al., 2006 and
subsequent studies)
• Reconstructs 2D coherent field
and flow structures from data
collected as structures cross
spacecraft
• Single spacecraft technique,
adapted for multiple ‘cuts’ by
different spacecraft
• Recovered FTE structures
indicative of multiple X-lines
Sheath
Magnetosphere
Pole
Equator
Other observable differences
2. Similarly, the
axial direction
also differs
between the lefthand model and
the longer X-line
models
 Fear et al.
(2010, 2012b)
Magnetosphere
Magnetosheath
1. A key difference
between FTE
models is the
azimuthal scale
size
 Wild et al.
(2005),
Dunlop et al.
(2005, 2011),
Fear et al.
(2008, 2010),
After Russell & Elphic
(1978)
After Lee & Fu
(1985)
After Southwood et al
(1988)/Scholer (1988)
Other observable differences
1. A key difference
between FTE
models is the
azimuthal scale
size
particularly if
the MP shear
is high
 Fear et al.
(2008, 2010)
2. Similarly, the
axial direction
also differs
between the lefthand model and
the longer X-line
models
MVA on
‘draping’
signatures
Fear et al.
(2012b)
See also
Trenchi et
al. (in prep)
GS on ‘core’
signatures
Other observable differences
Magnetosphere
Magnetosheath
3. The Lee & Fu
(1985)
mechanism can
produce isolated
FTE structures,
whereas the other
two must produce
FTEs in pairs
(one connected to
the northern
hemisphere, and
one to the south)
After Russell & Elphic
(1978)
After Lee & Fu
(1985)
After Southwood et al
(1988)/Scholer (1988)
An FTE seasonal bias?
• Raeder (2006) simulation
– In presence of dipole tilt, FTEs
were formed by multiple Xline reconnection
– FTEs moved preferentially
into the winter hemisphere
• Seasonal bias not present in all
subsequent simulations
• But if a seasonal bias is present in
spacecraft observations, suggestive
of mechanism that can produce
individual FTEs (i.e. not in pairs)
such as multiple X-lines
Raeder (2006)
Observational evidence
• Korotova et al. (2008) found
Interball FTEs near June
solstice observed exclusively
in winter hemisphere
• Effect verified with Cluster
data (different orbital
biases) – seasonal effect
needs to be taken into
account to understand
spatial distribution of events
(Fear et al., 2012a)
Korotova et al. (2008)
Lee & Fu (1985)
More complex topologies
• One puzzle with multiple X-line
model is why more complex
topologies are not observed
• Lee & Fu (1985) noted that in
addition to connections from
magnetosheath-magnetosphere,
could get:
– msh-msh, and
– msph-msph
Pu et al. (2013)
Connected to S
Connected to N
Closed MSPH
Closed MSH
More complex topologies
•
•
•
•
Plasma signatures often more complex than
simple picture
FTE substructure: mixture of RDs and TDs
Included some field lines with both footprints
connected to magnetosphere
Roux et al. (2015), “What is the nature of
magnetosheath FTEs?”, JGR, in press
Crater FTEs
•
FTEs are usually accompanied by an
enhancement in |B|, indicating magnetic
tension containing a pressure imbalance
(Paschmann et al., 1982)
•
However, many FTEs exhibit a more complex
‘crater’ signature in |B| (LaBelle et al., 1987;
Owen et al., 2008; Sibeck et al., 2008)
•
Farrugia et al. (2011) compared observations
from several Cluster instruments (FGM, CIS,
EDI, EFW, WHISPER)
•
Stratified structure best explained by single Xline picture
•
Electric field and electron observations in R2
evidence for encounter with separatrix (i.e.
field line mapping to reconnection site)
•
Suggested much progress can be made by
studying crater FTEs – “Highway to the Xline”
High cadence observations: Towards MMS
•
MMS provides a step change
in the cadence at which plasma
observations can be made
•
Previous instrumentation
usually limited by spacecraft
spin period
•
Varsani et al. (2014) exploited
Cluster capability to provide
sub-spin plasma observations
when B aligned with spacecraft
spin axis
•
Recovered structure reported
by Farrugia et al. (2011)
and others, but much greater
degree of subtleties…
•
Multiple layers of plasma
structure
•
Clear edges identified, but crossed in 0.5s – spin cadence observations would alias
Vortex-induced reconnection
•
Previous discussion in terms of
“2.5D” reconnection
•
Dorelli & Bhattacharjee (2009)
resistive MHD simulation
•
Argued that important to consider
3D reconnection separators etc
•
Interpreted FTEs as helical
magnetic fields rolled up by vortex
– reconnection caused by FTE, not
the other way round
•
Similar to Liu & Hu (1988)
mechanism, but considered fully
3D reconnection
•
Some observational studies do
consider fully 3D reconnection
concepts (e.g. Dunlop et al., 2009),
but relationship with conceptual
models still unclear
Open
(north)
Closed
Interplanetary
Open
(south)
Summary
•
•
Converging jets
(Hasegawa et al., 2010;
Øieroset et al., 2011)
•
Reconstruction results
(Sonnerup et al., 2004; •
Hasegawa et al., 2006)
•
Seasonal bias
(Korotova et al., 2008;
•
Fear et al., 2012a)
Evolution of ion
•
distribution and FTE
velocity [case study]
(Lockwood & Hapgood,
1998)
Crater FTEs (Farrugia et
al., 2011; Varsani et al.,
2014)
FTE velocity [stats]
(Fear et al., 2007)
MHD simulation
(Dorelli &
Bhattacharjee,
2009)
Summary
• FTEs potentially major driver of magnetospheric dynamics
(though order of magnitude discrepancy between spacecraft
and ionospheric estimates of flux transfer)
• Cluster/THEMIS have allowed significant progress, but still
debate about formation mechanism for FTEs
– Apparent conflict between some observations
• Reconciliation with truly 3D reconnection
• Cluster sub-spin observations show the potential for what
can be done with MMS