2013 THEMIS SCIENCE NUGGETS

First observations of foreshock bubbles upstream of Earth’s bow shock: Characteristics and comparisons to HFAs

by Drew L. Turner, UCLA IGPP


Introduction

At Earth’s quasi-parallel bow shock, where the angle between the interplanetary magnetic field (IMF) and the bow shock normal direction is less than ~45°, incident solar wind ions can be reflected and energized by the shock and subsequently travel back upstream. These counter-streaming suprathermal ions form a region of plasma known as the ion foreshock. Various kinetic instabilities resulting from the counter-streaming solar wind and foreshock populations result in enhanced ultra-low frequency (ULF) wave activity. For a review of the ion foreshock, see Eastwood et al. [2005]. The foreshock can also be responsible for generating large-scale transient kinetic phenomena, such as hot flow anomalies (HFAs), foreshock cavities, short large-amplitude magnetic structures, and foreshock cavitons. Recently, a new type of transient ion foreshock phenomena was predicted using global hybrid simulations: foreshock bubbles [Omidi et al., 2010].

Based on the Omidi et al. [2010] simulations, foreshock bubbles (FBs) can form just upstream of rotational discontinuities (RDs) in the IMF. As an RD moves through the foreshock, it can sweep up and concentrate some fraction of the hot foreshock ions on its upstream side. This concentration builds in time as the RD sweeps up more and more foreshock plasma on its approach to the bow shock. As a result of this concentration of these hot ions, the plasma expands to conserve total pressure, resulting in a region of decreased ion density and magnetic field strength. This hot, tenuous, low field strength region forms into the core of an FB. Due to the expansion, the local ion velocity within the core is also highly deflected from the unperturbed solar wind velocity and the IMF becomes significantly distorted. Around the core, a region of compressed plasma, exhibiting enhanced density and field strength, forms due to the expansion. On the upstream edge, this expansion into the incident solar wind can ultimately result in the formation of a fast magnetosonic shock. As the FB and this shock converge on the bow shock, since they both move upstream of the RD with the solar wind, this sets up an ideal scenario for particle acceleration due to first and second order Fermi acceleration. Figure 1a shows simulation results of an FB that formed in radial IMF on the dayside of the system. For comparison, a simulated HFA is shown in Figure 1b.

In Turner et al. [2013], we presented the first observations of FBs. These observations were made possible thanks to the unique, multipoint observations upstream of the bow shock provided by the THEMIS spacecraft. FBs have probably not been identified as a unique phenomenon before now due to their remarkable similarity to HFAs as observed in situ by a satellite. Due to these similarities, we identified seven characterization criteria for distinguishing between HFAs and FBs using single or multi-point spacecraft observations.

Figure 1. (a) 2.5-dimensional hybrid simulation results of a foreshock bubble [Omidi et al., 2010] showing the number density in color, normalized to the solar wind, and magnetic field lines. The X- and Z-axes are in units of the ion skin depth, where c is the speed of light and ωp is the ion plasma frequency. The axes are in simulation coordinates; the origin is located beyond the lower left corner of the simulation box. The white circle around the Earth represents the simulation’s inner boundary, a dipole inside a perfectly conducting sphere. A rotational discontinuity (RD) in the IMF, indicated on the figure where the IMF changed from purely radial (BIMF*[1, 0, 0] in XYZ) to having both an X and a Y component (BIMF* [1, –0.5, 0] in XYZ), results in formation of a foreshock bubble (FB) exhibiting clear core and upstream shock features. (b) 2.5-dimensional hybrid simulation results of a hot flow anomaly [Omidi and Sibeck, 2007] showing the number density in color, normalized to the solar wind, and magnetic field lines. The format is the same as shown in Figure 1a, but instead the HFA is related to a tangential discontinuity (TD) in the IMF.

Results

Figure 2a shows THEMIS-C (THC) observations of an FB that occurred on 14 July 2008. This was one of three FBs studied in detail in Turner et al. [2013]. As expected based on simulations, THC was initially in the ion foreshock as evident from the presence of suprathermal, counter-streaming ions. Just after 21:57:05 UT, THC observed a rotation in the IMF, and after this, the spacecraft observed a steady drop in plasma density and magnetic field strength. These features culminated in a region of very enhanced temperature that actually exhibited sunward plasma flows. The region also contained high levels of very energetic ions and electrons, and it was followed on the upstream side by a large compression region that terminated in a shock. Similar, though less extreme features were observed earlier upstream by THB. The timing between the two spacecraft indicated that the feature was moving with the solar wind. Assuming this type of motion, the event was ~7 to 9 RE in the XGSM direction. These features are all consistent with a foreshock bubble.

For comparison, an HFA observed by THC on the same day (14 July) is also shown in Figure 2b. Note that many of the features are similar, especially considering that the HFA shown has not fully evolved; HFAs can display features just as extreme. Based on the expectations from simulations of FBs and HFAs, we defined seven identification criteria to distinguish between FBs and HFAs. These are:

  1. HFA formation requires that an IMF discontinuity intersects the bow shock, but FB formation does not.
  2. HFAs form upstream, around, or downstream of tangential or rotational discontinuities, while FBs should only form upstream of some rotational discontinuities.
  3. FBs can grow to 10 RE or more in size and can form far upstream of the bow shock; HFAs are only a few RE in width normal to the discontinuity, form along the bow shock, and their features diminish rapidly within only a few RE away from the bow shock.
  4. FBs convect with the solar wind, but HFAs move along the bow shock with the discontinuity intersection.
  5. HFAs require that the electric field on one or both sides of the discontinuity be pointed back into it, while FBs do not.
  6. Except in the extreme case of a reflected-to-incident ion density ratio of more than ~65%, HFAs are bounded on both sides by compression regions, which may or may not form into shocks; FBs observed from within the foreshock should only be bounded on the upstream side by a compression or shock. However, when a spacecraft passes through the lobe of an FB from a location initially outside of the foreshock, it may observe the core bounded on either side by compression regions or shocks.
  7. The normal direction of the shock or compression region that forms on the upstream edge of an HFA can exhibit a range of orientations, while the normal direction of the shock or compression region on the upstream edge of an FB should exhibit a strong or dominant XGSM component.

FBs should be particularly effective at energizing particles since they involve strong wave activity and two converging shocks (i.e., the bow shock and that of the FB). Essentially, as the two shocks converge with a strong plasma wave environment between them, particles can be accelerated to high energies via first and second order Fermi acceleration. Figure 2c and d show differential energy flux distributions from four different plasma regimes observed by THEMIS on 14 July 2008: pristine solar wind, the ion foreshock, an HFA core, and an FB core. From these, it is clear that the FB resulted in the most energetic particles; the FB also completely thermalized the solar wind beam, which can also occur in extreme HFA cases [e.g., Zhang et al., 2010].

Turner et al. [2013] also conducted a preliminary survey of HFA-like and FB-like events from 14 July to 15 October 2008, corresponding to the period when THEMIS apogees were on the dayside of the magnetosphere. From this survey, 176 HFA-like events and 62 FB-like events were identified, with 45 additional events that could not easily be classified as either without further, more detailed analysis. When normalized by the coverage time in which both THB and THC were upstream of the bow shock (a criteria for event identification), this revealed that HFA-like events occur ~4 times per day, while FB-like events occur ~1 time per day. However, when those same results are examined for periods when the solar wind speed is above average (i.e., Vsw > 500 km/s), there are ~12 HFA-like events per day and ~5 FB-like events per day. This indicates that these events occur frequently. Since HFAs and FBs both involve significant pressure variations in the upstream solar wind, which can then penetrate through the magnetosheath and impinge upon the magnetosphere, these should be recognized as an important mechanism for perturbing the magnetosphere-ionosphere system.

Figure 2. Direct comparison of the first FB and HFA examples, including particle distributions from different plasma regions: (a) Enhanced view of the foreshock bubble observed by TH-C on 14 July 2008. (b) Enhanced view of the HFA observed by TH-C at ~22:28:30 UT on the same day. In both (a) and (b), characteristic regions and features are labeled. (c) Ion omnidirectional energy-flux distributions in the spacecraft frame for four different plasma regimes, all observed on 14 July 2008 by TH-C. Distributions are taken from the pristine solar wind between 22:50:30 and 22:51:00 UT, the ion foreshock between 21:59:45 and 22:00:15 UT, the core of the HFA shown in (c) between 22:28:30 and 22:28:45 UT, and the core of the FB shown in a between 21:58:00 and 21:58:15 UT. (d) Same as shown in (c) but for electrons.

Conclusion

With multipoint THEMIS data, Turner et al. [2013] showed the first observations of foreshock bubbles upstream of Earth’s bow shock. FBs have many similar characteristics to HFAs, so care must be taken in distinguishing between the two when identifying transient ion foreshock phenomena. Here, we defined seven characterization criteria that can be used to help distinguish between HFAs and FBs in single- or multi-point in situ observations. FBs and HFAs occur regularly upstream of Earth’s bow shock and should be important to energetic particle acceleration and impacts on the magnetosphere-ionosphere system. Furthermore, FBs should also form at other astrophysical shocks throughout the Universe and may play an important role in particle acceleration there as well.

Reference

Eastwood, J. P., E. A. Lucek, C. Mazelle, K. Meziane, Y. Narita, J. Pickett, and R. A. Treumann (2005), The Foreshock, Space. Sci. Rev., 118, 41–94.

Omidi, N. and D. G. Sibeck (2007), Formation of hot flow anomalies and solitary shocks, J. Geophys. Res., 112, A10203.

Omidi, N., J. P. Eastwood, and D. G. Sibeck (2010), Foreshock bubbles and their global magnetospheric impacts, J. Geophys. Res., 115, A06204.

Turner, D. L., N. Omidi, D. G. Sibeck, and V. Angelopoulos (2013), First observations of foreshock bubbles upstream of Earth’s bow shock: Characteristics and comparisons to HFAs, J. Geophys. Res. Space Physics, 118, doi:10.1002/jgra.50198.

Zhang, H., D. G. Sibeck, Q.-G. Zong, S. P. Gary, J. P. McFadden, D. Larson, K.-H. Glassmeier, and V. Angelopoulos (2010), Time History of Events and Macroscale Interactions during Substorms observations of a series of hot flow anomaly events, J. Geophys. Res., 115, A12235.

Biographical Note

Drew L. Turner is an assistant researcher within UCLA's Department of Earth, Planetary and Space Sciences. His research interests include energetic particles in space plasmas and space science instrumentation.

Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu