1. C. J. Farrugia1, J. G. Luhmann 3 , Jian Lan 4
Analysis of Suprathermal Proton Events Observed by STEREO/PLASTIC Focusing on the Observation of Bow Shock/Magnetospheric Events
J. A. Barry 1
, A. B. Galvin 1, M. Popecki 1, B. Klecker 2, H. Kucharek 1, K. Simunac 1,
C. J. Farrugia1, J. G. Luhmann 3 , Jian Lan 4
requirement is employed because particles thought to be associated with these events occasionally fall outside their defined time period. The histogram in Figure 4, utilizing optimal histogram binning [Scott et al., 1979], gives evidence that the possible Upstream/Magnetospheric events falloff sharply with STA-Earth separation. It is clear that the occurrence of events falls off quickly with spacecraft separation from the Earth. This result is expected since both magnetospheric and upstream events are field aligned and the target, the Earths bow shock, is becoming increasingly more unlikely to be connected to the spacecraft as the longitudinal separation increases.
In order to determine if the spacecraft is magnetically connected to the Earth’s bow shock we have developed a simple model. We assume that the spacecraft has a better possibility to be connected to the Earth’s bow shock if the solar wind velocity measured at the spacecraft falls between the solar wind velocity required for the spacecraft to be connected via a Parker spiral to a point 50Re upstream and 200Re downstream of the Earth along the Sun-Earth line (see
Abstract
The topic of suprathermal and energetic ion events upstream of the Earth’s bow shock has been investigated since the late 1960’s. Over the past 50 years these events have been characterized as having energies ranging from just above the solar wind energies on up through 2MeV, time spans of minutes to hours, and particle distributions ranging from field aligned to isotropic. The seed particles of these events accelerated within the magnetosphere and/or at the Earth’s bow shock have been shown to be of ions originating in the magnetosphere, solar wind, as well as ions energized in other heliospheric processes (such as Solar Energetic Particle (SEP), Corotating Interaction Regions (CIRs), Pick-up ions, etc.). In this study we utilize the particularly quiet solar minimum of 2007 to 2010 and the unique orbits of the STEREO spacecraft, STEREO-A(B) drifting ahead of (behind) the Earth in its heliocentric orbit at ~22º/year, to examine bow shock/magnetospheric energetic proton events in the region far upstream of the Earth’s ion foreshock. To do this, we first employ an automated procedure to identify suprathermal proton events in the energy range of 4keV up to 80keV with the ion composition instrument, STEREO/PLASTIC. These events are further classified as being associated with Stream Interaction Regions (SIR), Interplanetary Shocks, Interplanetary Coronal Mass Ejections (ICME), SEPs, or the Bow Shock/Magnetosphere. The Compton-Getting transformed energy spectra, ecliptic directionality (isotropic or field aligned), and magnetic connection to the Earth of 42 possible STA Bow Shock/Magnetospheric events are investigated as a function of the STEREO-Earth separation.
Introduction
Over the past 50 years of observation and modeling the sources of suprathermal ions/electrons upstream of the Earth’s bow shock, the sources are thought to be of solar wind, remnant processes, or magnetospheric origin [Asbridge et al., 1969; Sarris et al., 1976]. Solar wind ions can be accelerated in the region of the Earth’s bow shock to form upstream events of two different classes [Pashmann et al., 1981]. Field Aligned Beams (FABs) are beams of ions up to ~10 keV, formed from ions accelerated by Shock Drift Acceleration (SDA) at the quasi- perpendicular bow shock (see
event times is :
This indicates the exponential form fits the spectra only slightly better than the power law. Since the spectral index of the power law fit showed little dependence on STA-Earth separation, the average and 1 spectral index was calculated to be:
The decay constant in the exponential fit did change slightly with increasing STA-Earth distance as seen in Figure 7. At this time, the softening of the energy spectra with distance (and time) is still being examined.
Figure 3). The diffuse ion population [Ipavich et al., 1981a] is composed of ions accelerated via Fermi acceleration into an isotropic ion distribution with energies over 150keV/q. Within the ion foreshock [Ipavich et al. 1981b; Trattner et al., 1994] found an e-folding distance of ~10Re describes the falloff of the diffuse ion flux with increasing distance from the bow shock. Magnetospheric ions are shown to be accelerated in the magnetotail, magnetosheath and cusp regions [Anagnostopoulos et. al., 2005] and leaked into the upstream region with proton energies exceeding 300keV. These events are highly anisotropic.
In the region far upstream, beyond the ion foreshock, ions travel relatively scatter-free leading to highly anisotropic distributions [Scholer et al., 1981]. Observations by [Muller-Mellin et al., 2008; Klassen et al., 2009] using the STA/SEPT instrument confirm the existence of upstream/magnetospheric events at spacecraft-Earth separations of ~2000Re. At these distances the spectral from can be fit with a power law with spectral index of ~4, energies up to ~2MeV, have highly anisotropic distributions, and have last minutes to hours (dependent on magnetic connection to bow shock/magnetosphere).
Instrumentation
The STEREO mission is comprised of two identical spacecraft in heliocentric orbits. STEREO Ahead (STA) has an orbital radius slightly less than that of the Earth while STEREO Behind (STB) has an orbital radius slightly larger than the Earth’s see (Figure 1). From a reference frame at the sun, STA drifts ahead of the Earth while STB lags behind the Earth by about 22° per year.
For this investigation we utilize the Wide Angle Partition without a Solid State Detector (WAP non-SSD) portion of PLAsma and SupraThermal Ion Composition (PLASTIC) [Galvin et al.,
Figure 5a). In Figure 5b we have plotted the solar wind velocity with the area between the green bands representing the limits to when we could possibly be connected to the bow shock for STA. The purple vertical lines represent the bin size used in Figure 4. Note: the drop off in event occurrence is correlated with the spacecraft being outside the possible connection region.
Conclusion
From suprathermal particle events have been automatically identified on STA using the PLASTIC instrument. As was expected, the event frequency decreased with increasing STA separation from the Earth. For the case of upstream events, this is expected because the source region, the Earth’s bow shock/magnetosphere, becomes increasingly more difficult to be magnetically connected to the spacecraft. Through a simple model we showed that the possibility of observing upstream events should fall off outside of a Spacecraft-Earth separation of approximately 2000Re. This upper limit agrees well with our observations (excluding the single point at 6000Re) as well as the results of [Klassen et al.]. The distribution of event normalized linear miss indicates that over 83% of possible upstream events showed a stronger magnetic connection to the Earth than the surrounding time. The relevance in the softening of the energy spectra (decreasing e-folding energy) with increasing STA-Earth separation is still being examined.
These results show that the classification technique we employ to identify upstream/magnetospheric events yields events we are reasonably confident in claiming originate near the Earth. Detecting these events at such large STA-Earth separations demonstrates how these energetic particles travel relatively scatter free in the heliosphere during the quiet solar minimum of 2007.
Analysis
We have formed an automated procedure to define and list suprathermal proton events with the WAP non-SSD. At the PLASTIC website lists of the STA/B suprathermal proton events can be found along with a detailed description of the event definition. The STA events are categorized into groups depending on whether the event time span overlaps a SIR, ICME, SEP, or shock. The level 3 lists at the UCLA-STEREO page are used to determine the SIR, ICME, SEP and shock times. To obtain a list of possible upstream events we require that an event not overlap +(-) 1 day of the end (start) time of a SIR, ICME, SEP, and shock. This yields 60 possible upstream events of the total 269 events. This additional
2008] instrument and the In-Situ Measurement of Particles and CME Transients (IMPACT) [Luhmann et al., 2007] onboard STA to examine suprathermal protons in the energy range ( keV) for the existence of suprathermal particle events. The exclusion of a solid-state detector on the WAP non-SSD results in the resolution of particles to M/Q, which is sufficient for the detection of protons (Figure 2a). As in Figure 2a, the red oval in Figure 2b, a horizontal slice though PLASTIC, highlights the WAP non-SSD. The 180º in-ecliptic non-sunward facing look direction of the WAP non-SSD is broken up into 8 position sectors. This section of the instrument is ideal for identifying bow shock/magnetospheric ion events since it detects ions streaming directly from the Earth. The earthward direction relative to the WAP non- SSD is indicated with arrows and dates.
The smallest distance between the Earth and a line extended from the spacecraft along the measured magnetic field yields the linear miss [Desai et al., 2000]. Figure 6 gives the distribution of the event linear miss over the daily average linear miss. The peak of this distribution being < 1 indicates that during event time periods the spacecraft has a better magnetic connection with the Earth than throughout the rest of the day.
Due to low statistics when producing spectra, we combine upstream/magnetospheric events with < 60min between events. This reduces the number of possible events to 42. For each of the 42 events we obtain the spectra from the WAP non-SSD by first subtracting a background (time period with no events). We then transform the spectra into the plasma frame from the spacecraft frame via a Compton-Getting transformation. We then fit each position pixels spectra with a power law and an exponential. The average and 1 of the ratio of the coefficient of determination (R^2) between the two fits is for all positions and all
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1 EOS, University of New Hampshire
2 Max-Planck Institut für extraterrestrische Physik
3 Space Sciences Laboratory, University of California, Berkeley
4 NASA, Goddard Space Flight Center
This work was funded by NASA STEREO under contract NAS