by Jonathan Eastwood


One of the surprising results from the ISEE and AMPTE missions was the discovery that from time to time, one observed intervals of hot, highly deflected plasma flow near the bow shock. These structures were initially called ‘funnies’ (because of their strange appearance in the data), but became known as ‘Hot Flow Anomalies’. A variety of mechanisms were put forward to account for their formation, but a combination of theory, observation and numerical modeling showed that they are created by the interaction of a solar wind magnetic field discontinuity with the bow shock. A very useful review of the early years of HFA research is provided by Schwartz et al. [J. Geophys. Res. 105, 12639-12650, 2000].

HFAs are caused by solar wind discontinuities.
Figure 1. HFAs are caused by solar wind discontinuities.
Click here to enlarge the image.

In their review, Schwartz et al. noted that in the mid 1990s, ‘With theory and observation in accord, and the small, transient, apparently inconsequential nature of HFAs universally believed, the subject was considered closed.’ However, in a series of papers in 1998 and 1999, it was discovered that the pressure pulse set up by an HFA can cause the magnetopause to rapidly move by as much as 5 Earth radii from its initial location [Sibeck et al., J. Geophys. Res., 104, 4577– 4593, 1999], creating significant disturbances within the magnetosphere.

This is important because the solar wind structure that causes an HFA is fairly unremarkable – a magnetic discontinuity. One is led to the conclusion that magnetospheric activity that occurs when there is nothing of note in the solar wind could be due to the processing the solar wind undergoes at the bow shock. This has prompted renewed research into HFAs and other non-linear effects that could have a magnetospheric impact.

THEMIS is the perfect tool for these problems, because with five spacecraft orbiting the Earth, and a huge array of detectors on the ground, we can track these events from start to finish. Here, we describe THEMIS observations of an HFA from the cruise phase of the mission, which have recently been published in Geophysical Research Letters [Eastwood et al., Geophys. Res. Lett., 35, L17S03, doi:10.1029/2008GL033475, 2008]

THEMIS Observations

On 4 July 2007, the THEMIS spacecraft encountered an HFA. Uniquely, the HFA was captured on both sides of the bow shock; THEMIS-A was upstream, in the solar wind, and the other THEMIS spacecraft were in the magnetosheath. Furthermore, the THEMIS ground-based observatories saw the impact of the HFA on the magnetosphere.

THEMIS Hot Flow Anomaly, 4 July 2007.
Figure 2. THEMIS encountered an HFA on 4 July 2007. The spacecraft
were on the dusk flank of the bow shock, and the ground-based
observatories were aligned on the dawn side of the Earth.
Click here to enlarge the image.

Crucially, these observations allowed us to study the way in which the HFA disturbance is transmitted through the shock. Figure 3 shows the data from THEMIS-A (Upstream) side by side with the data from THEMIS-E (Downstream). THEMIS-A saw the classic signatures of an HFA between the two black vertical lines. Panel F shows the ion temperature – it peaks at 2000 eV, and panel E shows the three components of the velocity – the flow is strongly deflected. A more careful analysis shows that this structure fits all the requirements for an HFA. Note that the density (panel D) is much lower inside the HFA and enhanced on the edges. This means that the solar wind ram pressure is reduced in the center, and increased at the edges, so the magnetopause, in dynamic equilibrium with the solar wind, will be pushed in and out accordingly. This density depression sits on top of the magnetic field discontinuity.

Now look at the THEMIS-E data on the right. It was behind the shock, in the magnetosheath, and saw a much more complicated structure. The density cavity, in region 2, is now separated from the change in the magnetic field (region 3), which itself has split into several new discontinuities. Furthermore, the HFA has set up leading and trailing waves. To understand all this, we must get into the details of collisionless shock plasma physics…but what THEMIS immediately shows us is that the HFA does not survive coherently through the shock. Panel H of the THEMIS-E data shows the variation in the solar wind pressure at the magnetopause; The HFA caused a series of pressure changes, which will make the magnetopause oscillate, and send waves throughout the magnetosphere.

THEMIS A HFA July 4, 2007 THEMIS E HFA July 4, 2007
Figure 3. The left-hand figure shows the data from THEMIS-A in the solar wind. The HFA occurred between the
two vertical black lines. The right-hand figure shows data from THEMIS-E in the magnetosheath. The HFA
is much more complicated downstream!

The next step is to examine the magnetospheric response. This was directly revealed by the THEMIS ground-based observatories. Figure 4 shows the horizontal component of the magnetic field measured at five of the THEMIS ground-based observatories [Mende et al., 2008]. These five observatories all lie at a common geomagnetic latitude (60°N) and span 87° in geomagnetic longitude. At the time of the HFA, the magnetometers were arranged on the dawn flank of the magnetosphere as shown in Figure 2. The ‘wave’ (a magnetic impulse event) was observed to propagate across the chain away from the bow shock in the expected direction.

THEMIS Ground-Based Hot Flow Anomaly, 4 July 2007
Figure 4. Observations from the THEMIS ground-based
observatories. The wave in the magnetic field is a
magnetic impulse event set up by the HFA, which
propagated across the array from east to west, as shown
in Figure 2.
Click here to enlarge the image.

The observations from THEMIS are the first coordinated observations essentially tracking a Hot Flow Anomaly from the cradle to the grave. They have provided new insights into the basic plasma physics that controls HFA formation at the bow shock, and new insight into their magnetospheric impact.

One exciting development is the combination of THEMIS data with global hybrid magnetospheric simulations. Placing the ground truth of the THEMIS data in the context of the simulations will help to reveal the essential physics at work inside HFAs. A second development is the use of ground-based data from a variety of locations all over the world to understand the global impact of HFAs. We are currently comparing the northern-hemisphere THEMIS data with the southern-hemisphere Antarctic data, and hopefully this work will appear in a future nugget!

Biographical Note

Jonathan Eastwood is a research physicist at UC Berkeley. In his research, he aims to understand the basic science that governs space weather, in particular the physics of shocks and magnetic reconnection.

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