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Millstone Hill Preliminary Results During 16 - 21 Feb 1999

Philip J. Erickson
Atmospheric Sciences Group
MIT Haystack Observatory
Westford, MA 01886
USA

March 23, 1999 (update: May 17, 1999)

Observation Mode

Millstone Hill ran in the topside-oriented POLITE 5 experiment mode. Available pulse modes were 410, 1000, and 2000 usec modes from the zenith, and 640 usec modes in several directions from the MISA.

This type of experiment allows observations of radar-derived meridional neutral winds, vector F region electric fields, and topside parameters (light ion composition, density).

Geophysical Activity During The Period

SUMMARY: Small disturbance on Feb 17 at 12-15 UT, but largest disturbed conditions commenced Feb 18 at 03 UT. Kp of 5-6 persisted until Feb 19 at 15 UT.

Notes on Fitting

Short (410 usec) pulses from the zenith, and all 640 usec pulses from the MISA, were fit using the standard INSCAL fitter which only allows for O+ and H+. Long pulses (1000 usec and 2000 usec) from the zenith were fit with a specialized fitter employing constraints and a three-ion model with O+, H+, and He+.

There is significant He+ in the lower topside evident in this data. As a check that this fitted He+ is not actually H+, the following two figures show data vs theoretical spectra for two times and heights. The solid curve includes only O+ and H+, while the dashed curve is a full three-ion fit. The He+-inclusive fit does a better job matching the data (similar to Fig 1 of Gonzalez and Sulzer, GRL 23, 2509, 1996).


Click here for Postscript.


Click here for Postscript.

Millstone Hill Results

F Region Results (410, 640 usec pulses):

SUMMARY: Disturbance from Feb 17-18 causes high electron and ion temperatures, which drive a large depletion in electron density. A very sharp drop in NmF2 at 20-21 LT on Feb 18 (unlike days before or after), and increase in hmF2 occurs as the ionospheric trough moves to a full 7 degrees south of Millstone, exposing it to light ion trough conditions. Full recovery to daytime peak levels on Feb 16 is accomplished in only 2 days; by Feb 20 day, things are back to normal.

Entire Experiment Summary, Basic Parameters:

Individual Day Plots, Basic Parameters: SUMMARY: During Feb 18, Millstone is exposed to direct high-latitude flow patterns: eastward during pre-noon, westward during the afternoon and evening sectors. Northward electric field reaches 30 mV/m on Feb 18 afternoon, and westward velocity reaches 600 m/s. O+ ion outflow surge does not show up in parallel velocity at 300 km altitude, though. (Topside data must be examined for such a surge.) There is a clear southward velocity surge at 20-21 LT which is responsible for moving the trough past overhead at Millstone. Whatever caused this may also cause instabilities at Arecibo, airglow, etc. etc.

Winds show significant altitude variations on the initial disturbance night of Feb 18, with higher altitudes having larger flows (consistent with profile expanding upwards). Feb 18 evening and Feb 19 morning are quite variable compared to the quiet periods before and after. Radar-derived winds are in excellent agreement with FPI for Feb 20 - 21 night.

Derived Electric Fields, Vector Velocities, and Winds:

Expanded View of Electric Fields, Vector Velocities, and Winds for 18 LT Feb 18 - 06 LT Feb 19:

Topside Results (1000, 2000 usec pulses):

1000 usec Pulse

SUMMARY: Topside heating on Feb 18 is clearly seen, as is the depletion in electron density caused by expansion of the profile upwards. During the 20-21 LT period on Feb 18, there is some hint of a drop in He+ concentration corresponding to the deep trough overhead Millstone, although this length pulse cannot easily resolve things in the low density region. (See 2000 usec pulse below for better results.) In fact, the entire Feb 18 disturbed day is marked by much higher O+ fractions in the topside. These are characteristic of the light ion trough.

Entire Experiment Summary, Basic Parameters:

Individual Day Plots, Basic Parameters: Field-aligned velocity results TBD.

2000 usec Pulse

SUMMARY: The longer 2 msec pulse allows us to see more easily the signatures of O+ ion outflow in the trough brought overhead (and beyond) Millstone: a clear drop in He+ fraction on Feb 18 at 20-21 LT, along with Te heating. The entire Feb 18 day is marked by larger O+ fractions (nearly 100% below 1000 km) and lower densities, as the heated profile expands upwards and remains so. The O+/H+ and O+/He+ transition heights are well above the 1100 km limit of MHO observability throughout the period.

Entire Experiment Summary, Basic Parameters:

Individual Day Plots, Basic Parameters: Field-aligned velocity results TBD.

Supporting Information

DIGISONDE NETWORK SUMMARY: Digisonde network shows large depletions in peak electron density at Bermuda on Feb 18, Wallops and Goose Bay on Feb 18 and 19 similar to MHO observations. Puerto Rico sees large f0F2 fluctuations on Feb 18. hmF2 values show peak has moved to higher altitudes during periods of decreased density.

Digisonde Network:

DMSP SUMMARY: DMSP passes through North American longitude sector confirm significant structure on Feb 18 and 19 coincident with disturbance period. In particular, Feb 19 02 UT F12 and F14 passes show depletions in density extending down to 35 deg latitude, agreeing with MHO onset of large overhead density drop at 20-21 LT (01-02 UT). Large positive velocity signatures seen during this period.

All DMSP Satellite Passes Near 288 E longitude (data courtesy Fred Rich):

Examination of the 20-21 LT Feb 18 (01 - 02 UT Feb 19) passes for a time history of the trough movement southward TBD.
Philip Erickson <pje@haystack.mit.edu>
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