2010 THEMIS SCIENCE NUGGETS

Polar UVI and THEMIS GMAG observations of the ionospheric response to a hot flow anomaly

by Matt Fillingim

Introduction

Hot Flow Anomalies (HFAs) are disturbances in the solar wind flow caused by the interaction of a solar wind current sheet with the bow shock. They are characterized by an increase in the plasma temperature accompanied by a significant deflection of the plasma velocity. The solar wind dynamic pressure is reduced inside the HFA which leads to a deformation of the magnetopause. Magnetopause motions of up to 5 RE have been observed for some HFA events. Such large magnetopause changes have ionospheric effects. Ionospheric signatures of field aligned currents and even dayside auroral emissions have been associated with HFAs. Here we present global-scale observations of the ionospheric and auroral response to an HFA. Using the THEMIS Ground Based Observatory magnetometers (GMAGs) in conjunction with Antarctic magnetometers and photometers and global auroral images from Polar UVI, we are able to track the motion of the response over 6 hours of local time or 3000 km in the auroral ionosphere for the first time.

Observations

Previously, Eastwood et al. [2008] reported THEMIS observations of a hot flow anomaly at the bow shock and the resulting disturbance in the ionosphere. Here we expand upon the ionospheric and auroral response to the HFA. Figure 1 shows the three components of the ground level magnetic field measured at Goose Bay (GBAY) in eastern Canada. This signature is associated with a Hall current loop generated by a downward field aligned current poleward of GBAY. This field aligned current is caused by the magnetopause deformation resulting from the reduced dynamic pressure in the HFA.

Figure 1. From top to bottom: the magnetic north (H), magnetic east (D), and vertical down (Z) components of the local magnetic field observed by the THEMIS GMAG station at Goose Bay (GBAY) in eastern Canada. The 30 minute average has been subtracted.

Click each image to enlarge.

Figure 2 shows the H (north-south) component of the ground level magnetic field observed by six THEMIS GMAGs in the northern hemisphere and two ground magnetometers in Antarctica (SPA and MCM). The field aligned current signature is seen to propagate around the dawn side of the ionosphere from a local time of 7 to a local time of 1. The positive H deflections at higher latitude stations (SPA, MCM, and RANK) suggest than the field aligned current is equatorward of these stations at a latitude of about 70 degrees.

Figure 2. The H-component magnetograms for the 9 ground stations used in this analysis. The 30 minute average has been subtracted from each trace. The horizontal dotted line indicates the zero line for each trace. The spacing between adjacent lines is 100 nT. The time of maximum H deflection on each magnetogram is indicated by the vertical dotted line.

Click each image to enlarge.

The vertical dotted lines mark the times of maximum H deflection at each station. From the positions of the stations and the times of maximum H deflection, the velocity of the magnetic signature is 1.8 hours of local time per minute ±20% over 6 hours of local time. This corresponds to a velocity about 17 ±3 km/s over 3000 km at 70 degrees latitude in the ionosphere.

Figure 3 shows global auroral images from Polar UVI from the southern hemisphere. (However, the image projection is such that the observer is looking through Earth from above the north pole.) The first image shows only diffuse, low level emission on the dayside. Also shown in this image are the positions of the ground stations. The stations in red are the Antarctic stations; the stations in blue are the northern hemisphere THEMIS GMAGs mapped to the southern hemisphere.

Figure 3. A sequence of 4 global auroral images taken by Polar UVI over the southern hemisphere. The magnetic footprints of the ground stations are shown on the first image.

Click each image to enlarge.

The aurora begins to brighten at 10:38:30 UT near 8 local time at a latitude of 75 degrees as circled in the second image. (The white region in the second image between 18 and 24 local time at latitudes less than 70 degrees is missing data.) In the third image, the aurora has brightened significantly, the region of emission has grown in latitude and local time, and the center of the emission has moved anti-sunward to about 7 local time. The emission in the pre-noon sector has decreased to pre-event levels while the region of enhanced emission has moved past dawn to about 4 local time in the fourth image.

By using every available image and by binning the image data into 1 degree magnetic latitude bins and 0.2 hour local time bins, we can construct a local time keogram as shown in Figure 4. This keogram shows the auroral intensity as a function of UT and local time averaged over a fixed latitude range, 60 to 80 degrees magnetic latitude in this case. The horizontal dashed line marks midnight; the vertical dotted lines mark the times of the images shown in Figure 3.

Figure 4. Auroral intensity as a function of UT and magnetic local time averaged over magnetic latitudes from 60 to 80 degrees as observed by UVI. The plus signs show the UT and local times of the maximum H deflections observed by the ground stations. The lines show the propagation speeds of the magnetic signature (blue) and the region of auroral emission (red).

Click each image to enlarge.

Also shown on the keogram are the UT and local time positions of the ground stations when they observed the maximum H deflections. The times and locations of the northern hemisphere THEMIS GMAG stations are represented by the blue pluses; the time and locations of the Antarctic stations are represented by the red pluses. The least-squares best fit line is shown by the blue line.

The dayside auroral brightening is seen near the bottom of the keogram at a local time of about 6 around 10:40 UT. The red lines show the average propagation speed of the maximum auroral intensity observed by UVI. The average propagation speed of maximum auroral intensity from 10:38:30 to 10:43:24 UT is 0.29 hours of local time per minute or about 2.7 km/s at 70 degrees in the ionosphere, over six time slower than the propagation speed of the magnetic signature. At 10:43:24 UT, there appears to be a jump in the location of the maximum of the emission from 6 to about 3.5 local time. From 10:43:24 to 10:47:05 UT, the region of auoral emission is nearly stationary.

It is interesting to note that there is a sudden brightening of the aurora on the nightside near 23 local time at 10:42:47 UT. The timing of this brightening appears to coincide with the arrival of the magnetic signature to this local time. We leave open the possibility that the magnetic disturbance initiated a substorm once it reached the night side.

Conclusions

Using the THEMIS GMAG array in conjunction with Antarctic magnetometers and auroral images from Polar UVI, we have been able to track both the magnetic and auroral response to a hot flow anomaly over 6 hours of local time or over 3000 km in the auroral ionosphere for the first time. Our main result is that the propagation speed of the magnetic signature is 6 times faster than the propagation speed of the region of auroral emission. Since the velocity of the magnetic signature was calculated using mostly northern hemisphere stations while the aurora was observed in the southern hemisphere, the difference in velocity could imply a decoupling of the response between the two hemispheres. This event occurred near northern summer solstice so that the ionospheric conductivity in the northern hemisphere was much higher than that in the southern hemisphere. Perhaps the large difference in ionospheric conductivity may be an important factor in this decoupling. Alternatively, there could be decoupling of the field aligned current signature and the aurora implying that the dayside auroral response is not current driven. Further observations of the auroral and ionospheric response to HFAs are clearly needed to adequately address this issue.

Source

M.O. Fillingim, J. P. Eastwood, G.K. Parks, V. Angelopoulos, I.R. Mann, S.B. Mende, A.T. Weatherwax (2010), Polar UVI and THEMIS GMAG observations of the ionospheric response to a hot flow anomaly, J. Atmos. Sol.-Terr. Phys., 73, 137-145, doi:10.1016/j.jastp.2010.03.001.

Biographical Note

Matthew Fillingim is a research physicist at the Space Sciences Laboratory at the University of California, Berkeley. His current research interests include causes of changes in dayside aurora, modeling the ionosphere of Mars, and the plasma environment at the Moon.


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