1. Auroral Precipitation as a Driver of Neutral Density Enhancement in the Cusp
F. Brent Sadler1, A. Otto2, Marc R. Lessard1, Eric Lund1, H. Lühr3 , J. Sigwarth4
1 Space Science Center, University of New Hampshire, Durham, NH.
2 University of Alaska, Fairbanks, AK.
3 GeoForschungsZentrum (GFZ), Potsdam, Germany. 4 NASA Goddard Space Flight Center
AGU Chapman Fairbanks, AK Feb 28, 2011
Abstract
2. Cusp Enhancement Causes
4. CHAMP and FAST Data
5. Auroral Precip. Model Results
6. Cusp Aurora Observations
600 km
0 km
100 km
Joule heating
Electron precipitation, waves
CHAMP at 400 km --
accelerometer data
One idea…
Another idea…
Recent observations have confirmed neutral density enhancements at high latitudes which are localized to the polar cusp region. The small-scale structures associated this phenomenon are consistently correlated with strong small-scale field-aligned currents and are often associated with soft electron precipitation similar to that which drives night-side aurora ("auroral precipitation"). We investigate this issue with a numerical model originally developed to study dynamics associated with precipitation of tall auroral rays. The model incorporates detailed electron, ion and neutral dynamics to study various processes (e.g., heating, ion outflow, auroral luminosity) in a general sense: no mechanism is explicitly included to accelerate particles upward. Field and particle data from FAST and accelerometer data from CHAMP from a single favorable conjunction alignment event are input to the model. Results are given which support auroral precipitation as a driver to the density enhancement for this event. This mechanism requires a "cooking time" of 10 to 30 minutes before the density enhancement achieves steady state. Model results are compared with measured data from CHAMP for the event. Auroral images from the Polar satellite are used to further evaluate the model's results.
The CHAMP data from this event is presented in the following plots (Lühr). In the top panel we find a peak in neutral density. This is a common feature for the cusp/cleft region, as has been shown in several studies. The electron density exhibits a minimum at that time while the electron temperature peaks here. This is also quite typical for the cusp/cleft according to Proelss (2006, JGR). It also fits the idea of an ion outflow.
The two bottom panels show the FACs. We find bursts of kilometer-scale FACs (Rother et al., 2007, AG, 25/1603) occurring at the same time as the density peak. At the same time intense Large-scale FACs (< 150km) are observed. Positive values denote upward currents.
The plots shown (Otto) give typical evolution of plasma and neutral changes resulting from of ionospheric simulation of low energy particle precipitation. Plots give a one-dimensional profiles of the thermospheric structure over a 7 minute time evolution. The particle energy flux is assumed to be 4 mW/m2 and characteristic particle energy is 150 eV. The snapshots are take about 25 seconds apart (the isolated solid line always indicates the first response).
In the Auroral Precipitation Model, neutral density variations are initiated by auroral precipitation electrons which heat ions over extended periods of time; a "cooking time" of at least 10 to 30 minutes is needed. Since these electrons also cause aurora, observing aurora in the cusp over a period of time should help to indicate if this cooking time is being achieved prior to an observed density enhancement.
The phenomenon of the density increase of thermospheric gas in the Earth’s magnetic cusp was reported by Lühr et al. [GRL 2004] and was thought by those authors to be due to small-scale Joule heating in the cusp. Since that report was published, however, several other ideas have emerged which offer plausible explanations for this phenomenon.
150 eV
The above plots present electron number density, electron temperature and electron pressure. The time series start first with the slightly odd shaped solid line and then continue with dotted, dashed, dash-dotted to solid again. Remarkable are the large initial increases in density, electron temperature and pressure. The pressure increase generates associated gradient forces which accelerate the plasma.
1. Neutral Density Enhancements At High Latitudes km
200+ km km
Altitudes:
One of the earliest observations of neutral density enhancements at high latitudes is that reported by Jacchia and Slowey [JGR 1964], based on measurements of atmospheric drag on the Injun 3 satellite. They report increases in drag by a factor of 2 or 3 and associated with a KP = 6–. At this early date they explain that this effect is consistent with Joule heating as described by Cole [AJP 1962].
More recently, results from the CHAMP satellite confirm these and other observations of neutral density variations at high latitudes. Of particular interest are observations acquired with its ultra-sensitive accelerometer, reported by Lühr et al. [GRL 2004] and reproduced here.
The above Polar satellite plots from NASA’s CDAWeb are for the modeled event. These plots, one each with geographic and magnetic projections, show a time sequence of aurora in the vicinity of the density enhancement (red circle). They show that aurora is indeed active in the general region of interest for a period preceding the observation of the density enhancement. A plot of the true image perspective (inset) indicates that Polar has a fair but far from ideal position for viewing aurora for this event.
Auroral observations of this type will help us better understand "cooking time". Our next step is to investigate cusp aurora from calibrated ground imagers (Svalbard, Greenland, etc.) which are are more likely to give better resolution and viewing angle. In addition, the Auroral Prediction Model is capable of predicting auroral intensity. Calibrated imagers could help validate this aspect of the model as well.
Strangeway, et. al., 2005 propose two sources for ion outflow: Poynting flux (Joule dissipation) and electron precipitation (electron heating / ionozation). Neutral density enhancement may be caused by more than one mechanism as well.
150 eV
Very remarkable upward ion acceleration, up to 2.5 km/s in about 3 minutes. Neutrals are effected through the drag of the ions. Neutral velocities are much smaller (as expected) but reach about 8 m/s at altitudes of ~400 km and about 23 m/s in the upper thermosphere. An additional aspect in the neutral motion is the heating of the neutrals through energy exchange (and charge exchange) collisions which further thermal expansion in the F region.
Neutral density changes are caused by 2 mechanisms: (a) upward lifting of neutral material transports denser material to greater height. (this is quantitatively approximately consistent with the observed density changes.). (b) divergence of the upward velocity leads to compression or expansion of the corresponding material. Both mechanisms are about equally important.
Corresponding FAST data, shown below were acquired within 2-5 minutes of the CHAMP crossing, show plasma measurements in this same region, roughly an hour (in MLT) to the west. The top panel shows magnetic field data, indicating strong field-aligned currents near 1611 UT, or near 75º ILAT, nearly the same latitude as seen by CHAMP. The second panel shows electron energies, the third panel shows electron pitch angles, the fourth panel shows ion energies and the fifth panel shows ion pitch angles.
Aside from the field-aligned currents, the notable features in these data are the precipitating electrons at 1611 UT, having energies ranging from thermal to hundreds of eV and pitch angles that are fairly isotropic except for a loss cone.
3. Feb 5, 2003 Conjunction
To provide data for the numerical models, we take data from two satellites, CHAMP and FAST, in conjunction over the dayside cusp region. Numerous conjunctions have been identified as having promise.
6. Conclusions
This figure shows drag measured by the accelerometer on CHAMP, with the vertical axis showing deceleration and the horizontal axis showing time of day. The narrow peaks, however, indicate “bumps” observed as the satellite passed over the cusp region. (The large scale variations show orbital fluctuations not associated with the cusp deceleration.)
When observing these “bumps”, CHAMP simultaneously measured strong field-aligned currents, an indication that the density enhancement in the auroral zone may arise from fundamentally different processes than what occur at lower latitudes.
The traditional relationship between ionosphere and thermosphere is coupling via Joule heating, where the thermosphere is simply heated by collisional processes. New ideas (A. Otto) are relating neutral enhancement to electrodynamic processes, such as those related to ion outflow. In this case, it may result from soft electron precipitation, Alfvénic waves or small-scale currents.
1. Data from a conjunction between the CHAMP and FAST satellites during a density enhancement event are input to an Auroral Precipitation Model.
2. The model gives a satisfying time evolution for various parameters at a single point along the CHAMP path.
3. Temporal auroral observations from the Polar satellite seem to indicate that aurora was present in the cusp region prior to the event observed by CHAMP.
4. This helps validate the model by indicating that appropriately long "cooking time" may have existed for this event.
Future work:
- Work with calibrated ground-based imagers near the cusp.
- Estimate energy input to the heating process.
- Compare model predictions of auroral intensity.
The first conjunction studied was on Feb 5, 2003, at approximately 1600 UT. The figures above show the conjunction, as presented by the NASA SSCWeb 3D Orbit Viewer tool. The red trace shows the FAST trajectory and the blue trace shows that of CHAMP. The left image shows a full orbit trace on this date. Clearly, their orbital planes are similar as they pass through the noon meridian. In addition, the spacecrafts move through this region within 2-5 minutes of each other, providing an excellent conjunction. The dots on the trace indicate the position of the satellites at 1555 UT.
The right two images show the partial trace of the satellites between 1555 and 1610 UT. The dots on these traces indicate the position of the satellites at 1603 UT.
300 eV
Increasing the characteristic energy from 150eV to 300eV decreases the neutral enhancement. The larger characteristic energy precipitation leads to deposition of the energy deeper in the thermosphere where the neutral inertia is much larger and friction forces prevent the plasma to accelerate efficiently.