Transcript Here is the Original File - University of New Hampshire

Pulsating aurora observed on the ground and in-situ by the Van Allen Probes
Marc Lessard1, Ian J Cohen1, Philip Fernandes5, Richard E Denton5, Mark J. Engebretson2, Craig Kletzing3, John R Wygant4, Scott R Bounds3, Charles W Smith1, Robert J MacDowall6, William S Kurth3
1. University of New Hampshire, Space Science Center, 8 College Rd, Durham, NH 03824 USA; 2. Augsburg College, Physics Dept. 2211 Riverside Ave. Minneapolis, MN. 55454 USA, 3. Department of Physics and Astronomy, University of Iowa, Iowa City, IA, United States.
4. School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, United States. 5. Department of Physics and Astronomy, Dartmouth College, Hanover, NH, United States. 6. Solar System Exploration Division, Goddard Space Flight Center, Greenbelt, MD, United States.
Introduction
This study is largely motivated by recent work (P. Fernandes, Senior Thesis) that shows persistent fine-scale structure in
pulsating auroral patches. This structure is typically observed to have sharp boundaries in the auroral forms. This is one
important point, since the patches must be associated with corresponding structures in the equatorial region. Of equal
importance is the fact that these well-defined forms can retain their shapes as the patches disappear and re-appear several
(and tens of seconds) seconds later.
In total, approximately 350 ± 10 minutes of pulsating aurora were recorded in conjunction with the ROPA rocket campaign
in Alaska in 2007. Patches pulsated with periods ranging from 1 – 20 seconds. During these pulsating aurora events, a total
of 26 distinct “black” auroral forms were observed. Such forms are often thought to be evidence of upward-moving electrons
that constitute return current regions.
All black auroral forms observed were very elongated, with their lengths typically extending an order of magnitude further
than their widths. Some events appeared to form a boundary between patches (as in the example below); in other events, a
back stripe appeared within a region of diffuse aurora.
Pulsating aurora, chorus, and poloidal waves
In a recent paper, Li et al., [JGR, 2011] show observational and theoretical results from the THEMIS spacecraft, where chorus waves were
modulated by Pc 4-5 pulsations. They attribute the modulation of chorus waves to variations in the ratio of resonant electrons to total electrons
associated with the Pc 4-5 waves.
In the example presented here, we show similar results acquired by the Van Allen Probes, with the additional observation of widespread pulsating
aurora at the footprints of the spacecraft. The observations in this case also differ form those in Li et al in the sense that chorus waves in this
example occur at half the period as the Pc 4-5 waves (which may be poloidal mode waves). The implication is that pulsating aurora may be
directly connected to Pc 4-5 pulsations (i.e., perhaps poloidal field-line resonances).
Two papers addressing poloidal mode waves are relevant to this work. One is a statistical study of ULF by Anderson et al. [JGR, 1990] that shows
occurrences of radially polarized waves in the post-midnight region. Occurrence rates of these waves are much lower than pulsating aurora
occurrences. On the other hand, the event shown here is clearly visible in the electric fields, but not the magnetic fields. The other paper that is
relevant is James at et al [JGR, 2013], showing observations of poloidal modes in conjunction with substorm injections. Taken together, these
results imply that substorm injections drive poloidal modes, which modulate chorus that can scatter the energetic electrons that produce pulsating
aurora.
Sponsored by NSF
The figure to the right shows VAP
data from 1215-1245 UT, an interval
where “monochromatic” Pc 4-5
pulsations are clear in the electric
field data (but not so clear in
magnetometer data). In the lower
two panels, bursts of chorus waves
are observed, with the period of
these bursts being precisely half of
the period of the Pc 4-5 waves.
Also, note that the intensity of the
waves approximately correlates with
the amplitude of the Pc 4-5 waves.
In addition to the correlation of Pc 45 waves with chorus waves, we
note that the duration of the bursts
is the order of several seconds, a
typical period for pulsating aurora.
An important implication: the
coherence and periodicity of the
bursts and their correlation with the
(large-scale) Pc 4-5 waves must
mean that spacecraft are observing
temporal effects, not spatial. The
shape of the patches in the
ionosphere must represent the
shape of the regions of chorus in
the magnetosphere.
Well, that’s just fine and dandy, but
what about the fine structure,
explained earlier?
The role of the ionosphere
Example of a black auroral form located at the boundaries of pulsating patches. The
pulsating patches above and below the form were observed to pulsate with different
frequencies. Indicated are the axes along which length and width were determined.
Brightness and contrast have been modified. Image was taken January 18, 2007.
One version of black aurora was observed during dynamic auroral activity and consisted of a black auroral form embedded
within a pulsating patch. Figure 18 displays an example of this type of form. When the patch was on, the black aurora was
observed within the patch. When the patch pulsated off, the black aurora was unseen. As the patch pulsated on and off, the
black auroral form maintained its shape and orientation. In the example shown, the form was observed embedded within the
patch for a duration of 2.4 minutes. In this case, the patch and the black aurora drifted across the field of view, from west to
east. Both the patch and the black aurora drifted with the same velocity. As the patch and the black aurora drifted from west
to east, the aft region of the black aurora came into the field of view, indicating that the black aurora extended beyond the
field of view of the camera.
The persistent shape of the signature in the ionosphere is easier to understand as an effect resulting from increased
ionization associated with auroral precipitation, rather than being a result of a well-bounded region in the equatorial region of
the magnetosphere. In particular, the enhanced ionospheric conductivity would only disappear through recombination, a slow
process. In addition to the persistence of well-bounded regions in the ionosphere,pulsating patches show distinctive structure
(often including black aurora, etc). Such small-scale features may be associated with ionspheric feedback.
The figure to the left shows an overview of VAP data, spanning 0800-1500 UT on 26 Jan 2013. The specific portion of the
orbit that mapped to pulsating aurora cannot readily be identified because of cloud cover over much of the THEMIS camera
array. However, we estimate that the spacecraft “entered” the auroral zone near 1000 UT and “exited” the region near 14:30
UT.
The role of the ionosphere in controlling auroral arcs has long been a debated issue: is the ionosphere passive or active?
Atkinson [1970] and Sato [1978] proposed theories in which convection across an E-region conductivity gradient causes
paired upward and downward field-aligned currents (FAC). The upward current induces energetic electron precipitation and
this precipitation modifies the conductivity, causing the gradients to grow. With larger conductivity gradients, the FAC grow.
Called the ionospheric feedback instability, this process has been considered by many authors in various forms.
The figure to the right (from the Atkinson paper) tells the story. An
assumed conductivity variation (b) produces electric field (c) in the
ionosphere, which maps to the magnetosphere as (d). A flux tube
flowing through the system sees a time-varying magnetospheric
electric field (e), causing polarization currents (f) that close by fieldaligned currents (g). The upward-directed current occurs as
precipitating electrons, which cause the conductivity variation (b)
and close the feedback loop.
The electric field measured by the VAP over pulsating auroral is
near zero (see plot to the left), consistent with the mechanism
described by Atkinson. Beyond this observation, concluding what
role feedback might play is not possible with these data.
Example of a black auroral form (black arrows) imbedded within a pulsating patch. In the first
frame, neither the patch nor the black form is visible. In the second frame, the patch is at full
brightness and the form is imbedded within it. Brightness and contrast have been modified.
Images were taken January 18, 2007.
The main conclusion here is that pulsating patches typically maintain well-defined shapes for many pulsating periods, even
throughout intervals where the pulsation activity appears to have ceased and then re-appears. That is, the basic shape of
the patches is well-defined and persistent. As these features, in principle, map to the equatorial region (where chorus waves
scatter the electrons that cause the aurora), we are led to the following questions:
1.What process(es) determines the size and shape of pulsating auroral patches? In principle, ionospheric feedback should
play some role, though details are not clear.
2.How can it be that the shape of the patches is maintained even after it seems to have ceased pulsating but then returns?
Do black regions represent return currents?
3.What controls the period of the pulsations?
Conclusion
The movie above shows allsky camera data from Poker Flat,
Alaska, with pulsating aurora beginning near 1130 UT (following
two substorms). Allsky camera data from Gillam, Manitoba,
showed pulsating aurora beginning at 0920 UT (visible as
clouds moved away from the area), recorded until sunrise
forced the camera to shut off at 1030 UT. A camera further west
(The Pas) also showed pulsating aurora beginning at 0900 UT,
filling the entire field-of-view of the camera and persisting with
varying brightness until the camera also shuts off at 1030 UT.
Jones et al. [JGR, 2013] showed THEMIS allsky camera
observations of widespread, persistent pulsating aurora that,
like this event, occurred across Canada and Alaska. This and
other work lead us to believe that such events are common and
that, more importantly, the Van Allen Probes pass through a
region of extended pulsating aurora.
The figure above shows the footpoint of VAP-A and B at
1157 UT. Pulsating aurora was widespread, occurring at The
Pas, Gillam and a Poker Flat
Observations from the VAP over a region of pulsating aurora lead to the following conclusion:
1.Bursts of chorus emissions in conjunction with pulsating aurora are well-correlated with Pc 4-5 waves (poloidal
resonance?). Such waves may be driven via substorm injections and likely causes the chorus (e.g., Li et al., 2011)
2.The association of chorus waves with Pc 4-5 waves (i.e., the periodicity and coherence of the bursts of chorus, in
particular) link them to the Pc 4-5 waves, which are large scale phenomena, even for high-m cases. A spacecraft
transiting the region at speed of a few km/s, therefore, must be embedded in an extended and relatively uniform region.
The modulations of these bursts is a temporal effect.
3.The shape of the patches in the ionosphere must represent the shape of the regions of chorus in the magnetosphere,
albeit possibly modified by ionospheric feedback.
4.Ionospheric feedback can be expected to play some role in the process, though it has not been considered theoretically
(for pulsating aurora) and is nearly impossible to observe experimentally. Still, the persistent shape of the patches and the
fine structure observed in the patches seems to be linked to ionospheric feedback processes.
Acknowledgement: Research at the University of New Hampshire was supported by NSF grants AGS-1202827 and PLR-114987
and NASA grant NNX13AJ94G