IntroductionIntroduction Pulsating aurora, chorus, and poloidal wavesPulsating aurora, chorus, and poloidal waves

Acknowledgement: Research at the University of New Hampshire was supported by NSF grants AGS-1202827 and PLR-114987 and NASA grant NNX13AJ94G

Pulsating aurora observed on the ground and in-situ by the Van Allen ProbesMarc 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.

Sponsored by NSF

The role of the ionosphereThe role of the ionosphere

ConclusionConclusion

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.

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 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?

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.

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 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 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

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 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 4-5 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 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 field-aligned 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.

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.

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.

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.

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.