Dwight P. Sipler

Observations from Millstone Hill during the geomagnetic disturbances of March and April 1990

The incoherent scatter radars at Millstone Hill operated continuously during the periods March 16–23 and April 6–12, 1990, providing observations of large-scale ionospheric structure and dynamics over a large portion of eastern North America. Major geomagnetic storms occurred during each of these periods, with deep nighttime ionospheric troughs and large magnetospheric convection electric fields observed equatorward of Millstone. The Millstone observations provide a comprehensive data set detailing storm-induced ionospheric effects over a 35° span of latitude during both of these intervals. At the latitude of Millstone the ionospheric peak height hmF2 rose above 600 km in the trough on March 22 and 23 and reached ≈500 km at night on April 11 and 12. Increased recombination, apparently due to the strong electric fields, the temperature dependent recombination rate coefficient, and neutral composition changes, greatly depleted the F2 region over a wide latitude range during the day on April 10, 1990. This resulted in an ionosphere dominated by molecular ions, with ionospheric peak heights below 200 km on this day. A number of frictional heating events during the disturbed periods are seen from comparison of ion temperature and velocity measurements. The most intense event took place near 1200 UT (≈0715 LMT) on April 10, 1990, when Kp reached 8. At 0110 UT on March 21, line of sight ion velocities in excess of 500 m s−1 were observed at the extreme southern limit of the Millstone steerable radars field of view (40° apex magnetic latitude at an altitude of 700 km). These could be due to penetration of magnetospheric electric fields or electric fields associated with ring current shielding in the storm-time outer plasmasphere. About an hour later, ion outflow was observed just equatorward of Millstone. This is most likely due to heating from a latitudinally confined region of intense westward convection. Neutral meridional winds above Millstone were obtained by three different techniques employing radar and Fabry-Perot measurements. The latitude variation of the winds was also estimated from radar measurements of hmF2 and electric fields using the servo model method. Strong equatorward nighttime neutral wind surges were found during both the March and April disturbances, which reached the equatorward limit of the observations at F peak heights.

Ionospheric effects of the March 1990 Magnetic Storm: Comparison of theory and measurement

This paper presents a comparison of the measured and modeled ionospheric response to magnetic storms at Millstone Hill and Arecibo during March 16-23, 1990. Magnetic activity was low until midday UT on day 18 when Kp reached 6, days 19 and 20 were quiet, but a large storm occurred around midnight UT on day 20 (Kp=7) and it was moderately disturbed (Kp=4) for the remainder of the study period. At Millstone Hill, the daytime peak electron density (NmF2) showed only a modest 30% decrease in response to the first storm and recovered to prestorm values before the onset of the second storm. The model reproduces the daytime peak electron density well for this period. However, the severe storm on March 20 caused a factor of 4 depletion in electron density, while the model densities were not greatly affected. The inclusion of vibrationally excited nitrogen (N2*) in the model was unable to account for the observed large electron density depletions afterward March 20. The storm did not appear to affect the overall magnitude of the electron density at Arecibo very much, but did cause unusual wavelike structure in the peak density and peak height following the storm. The model reproduces the daytime NmF2 very well for Arecibo, but after sunset the model densities decay too rapidly. This study indicates that successful modeling of severe ionospheric storms will require better definition of the storm time inputs, especially of the neutral atmosphere.

TIME DEPENDENT THERMOSPHERIC NEUTRAL RESPONSE TO THE 2-11 NOVEMBER 1993 STORM PERIOD

Abstract Many satellite and ground-based observations from 2–11 November 1993 werecombined in the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure toderive realistic time dependent global distributions of the auroral precipitation and ionosphericconvection. These were then used as inputs to the Thermosphere–Ionosphere–ElectrodynamicsGeneral Circulation Model (TIEGCM) to simulate the thermospheric and ionospheric responseduring the storm period. The November 1993 storm was an unusually strong storm associatedwith a recurring high speed stream of solar plasma velocity in the declining phase of the solarcycle. Significant gravity waves with phase speeds of about 700 m/s caused by Joule heating werepresent in the upper thermosphere as perturbations to the neutral temperature and wind fields,especially on 4 November. The observed gravity waves in the meridional wind and in the height ofthe electron density peak at several southern hemisphere stations were generally reproduced bythe model using the AMIE high latitude inputs. Both model and observed equatorward windswere enhanced during the peak of the storm at Millstone Hill and at Australian ionosondestations. The observed neutral temperature at Millstone Hill increased about 400 K during thenight on 4 November, returning to normal on 9 November, while the model increased 300 K thefirst night at that location but was still elevated on 11 November. Enhanced westward windsduring the storm were evident in the UARS WIND Imaging Interferometer (WINDII) data. Theenhanced westward winds in the model were largest around 40–45° magnetic latitude at night,and also tended to be largest in the longitudes containing the magnetic poles. The peak westwardwind enhancements at 0 LT reached about 250 m/s at 300 km, and about 100 m/s at 125 km thefirst day of the storm at 40° magnetic latitude. At 20° magnetic latitude, the maximum westwardwind enhancements at 125 km at 0 LT appeared 2–4 days after the major part of the storm,indicating very long time constants in the lower thermosphere. The model showed global averageneutral temperature enhancements of 188 K after the peak of the storm that decayed with time,and which correlated with variations 8 h earlier in the Dst index and in the electric potential dropinput from AMIE. The global average temperature enhancement of 188 K corresponded to apotential drop increase of only about 105 kV. The results showed that the TIEGCM usingrealistic AMIE auroral forcings were able to reproduce many of the observed time dependentfeatures of this long-lived geomagnetic storm. The overall global average exospheric temperaturevariation correlated well with the time variation of the cross-tail potential drop and the Dst indexduring the storm period. However, the enhanced westward winds at mid-latitudes were stronglyrelated to the corrected Joule heating defined by the time dependent AMIE inputs.

Combined optical and radar wind measurements in the F region over Millstone Hill

A Fabry-Perot interferometer has been gathering F region wind data at Millstone Hill routinely since mid-1988. A total of 27 nights of optical data have been collected with simultaneous incoherent scatter radar data. Data for 22 nights (of which 10 were geomagnetically quiet and 12 were unsettled) have been used to evaluate the O+-O collision frequency. Our data show that the collision frequency based on Dalgarnos adjusted theoretical value (Dalgarno, 1964) that was used at Millstone Hill Observatory before 1988 should be increased by a factor of 1.9±0.15.

Solar activity variations in midlatitude thermospheric meridional winds

Upper thermospheric meridional wind data at midlatitudes and for low magnetic activity are examined for solar activity variations following an analysis scheme suggested by a Coordinated Analysis of the Thermosphere workshop. Wind data from incoherent scatter, Fabry-Perot, and F2 peak heights show decreasing diurnal amplitudes with increasing solar activity during all seasons, except for Saint Santin data, which show a slight increase in summer. Equivalent winds from F2 peak height data have strong decreases in diurnal amplitude in all seasons. The coupled thermosphere ionosphere model and thermosphere ionosphere global circulation model predictions of diurnal amplitude, while differing considerably in magnitude, also show decreasing amplitudes during all seasons except summer, while the HWM90 empirical model amplitudes increase slightly with solar activity during all seasons. The diurnal mean wind trends with solar activity are fairly weak, except for Millstone Hill incoherent scatter radar, which shows a shift from strong southward to near zero or northward wind with increasing activity. Model results for the mean generally fall within the band of measurements. Near midnight, most of the data also show that the typically southward winds weaken with increasing solar activity in all seasons except summer, when results are mixed. There are significant differences between the trends and between absolute values for the various data sets and models which need further investigation.

Vertical winds in the midlatitude thermosphere from Fabry-Perot interferometer measurements

Abstract Monostatic and bistatic measurements of thermospheric winds have been made with Fabry-Perot Interferometers at the Millstone Hill and Laurel Ridge Observatories. Synchronized observing sequences have been chosen to enable the determination of vertical winds from the measurements. The vertical winds are found to be significant on some nights with velocities of tens of m/s. For geomagnetically quiet nights, the averaged values for eight samples in the July–October 1992 period show little variation during the night, averaging ∼10 m/s downward. The average for three geomagnetically disturbed nights oscillates from a downward maximum of ∼50 m/s at ∼02 UT to an upward maximum of ∼20 m/s at ∼07 UT. Temporal variations in the vertical motions suggest oscillatory behavior with periods of ∼0.7, ∼1.8 and ∼3.4 h, respectively, on three quiet nights, possibly associated with gravity wave or tidal-harmonic effects.

Estimation of the O+, O collision frequency from coincident radar and Fabry‐Perot observations at Millstone Hill

The formula for the O+, O momentum transfer collision frequency has been uncertain due to a discrepancy between results of theoretical calculations and some of the joint radar/optical studies. The former suggest a multiplicative factor equal to 1.2–1.3 times the formula derived by Dalgarno [1964] and Banks [1966], while the latter suggest a multiplicative factor F = 1.7 [Salah, 1993]. We present results of a new analysis of data from 30 nights of coincident incoherent scatter radar (ISR) and Fabry-Perot interferometer (FPI) experiments conducted at Millstone Hill between 1988 and 1992. The O+, O collision frequency is estimated from FPI measurements of the horizontal neutral wind in the magnetic meridian, ISR measurements of the ion drift velocity parallel to the Earths magnetic field and other data at the calculated height of peak 630 nm emission, and the mass spectrometer and incoherent scatter 86 model. A complete error analysis is carried out for each derived value of F. This allows us to carry out Monte Carlo simulations which confirm that random errors lead to an increase in the mean value of F and which provide us with an unbiased result, F = 1.15 ± 0.2. However, this result was obtained from an analysis which neglected vertical neutral winds, about which we have little information. The most likely effect of these winds would be an increase in the value of F, so that our best estimate from this study is F = 1.4 ± 0.3, which is consistent with theoretical calculations.

Comparison of models and measurements at Millstone Hill during the January 24–26, 1993, minor storm interval

Results from four first-principle models are compared with Millstone Hill incoherent scatter radar and Fabry-Perot interferometer measurements taken during January 24–26, 1993, a period which included a minor geomagnetic storm. The models used in this study are the thermosphere ionosphere electrodynamics general circulation model (TIEGCM) with and without forcings from the assimilative mapping of ionospheric electrodynamics (AMIE) technique, the coupled thermosphere ionosphere model (CTIM), and the field line interhemispheric plasma (FLIP) model. The present study is the first time the AMIE inputs have been used in the TIEGCM model. TIEGCM and CTIM both underestimate the neutral temperature because of an underestimation of the Joule heating rate. An increase in the high latitude Joule heating would modify the thermospheric circulation. This could result in increases in N2 and O2 density above Millstone Hill, which would decrease the AMIE TIEGCM peak electron density (NmF2) to agree better with the observations, but would result in poorer agreement between CTIM and the data. The FLIP model NmF2 is a little low compared to the data, perhaps because of an inadequacy of the mass spectrometer incoherent scatter (MSIS) 86 model composition or the H+ flux in the model. Good agreement is obtained between atomic oxygen density [O] given by MSIS and [O] obtained from the radar data using a heat balance equation, provided an O+–O collision frequency factor of 1.3 is used. While the TIEGCM underestimates the electron and ion temperatures, the FLIP model reproduces major features of the data, apart from a large nighttime enhancement in Te. During the minor storm interval the observed neutral winds show alternating equatorward surges and abatements apparently due to passage of traveling atmospheric disturbances (TADs) seen in the model results. These are associated with a late evening increase observed in NmF2 accompanied by a large increase in F2 peak height (hmF2). These perturbations in NmF2 and hmF2 are not reproduced by the TIEGCM or CTIM. The NmF2 increase may be due to a decrease in O+ recombination rate caused by the higher hmF2, combined with compressional effects of a TAD and an enhanced downward flux of O+ ions.

All-sky Doppler interferometer for thermospheric dynamics studies

An efficient, all-sky input optical system has been mated to a 100-mm-aperture Fabry-Perot interferometer that employs a cooled (-150 °C) CCD as a photon detector to create an all-sky Doppler interferometer. The instrument is capable of simultaneously measuring Doppler shifts and widths of nightglow emission lines from many different points in the sky, thereby providing determinations of upperatmosphere neutral wind and temperature fields over a large region (to ≈2000 km in extent). For OI 630-nm (thermosphere) and OH 799.6-nm (mesopause) nightglow emissions, exposure times of 5-15 min provide good-quality interferometric images. The capability of the all-sky Doppler interferometer is illustrated by examples of thermospheric wind and temperature fields measured over Millstone Hill, Massachusetts.

Rayleigh lidar observations of a mesospheric inversion layer during night and day

A narrow field of view Rayleigh lidar has been constructed at Millstone Hill / MIT Haystack Observatory (42.6°N, 71.5°W) for observations of middle atmospheric temperatures throughout the diurnal cycle. During a 31.5 h measurement on 19–21 March 2001 a mesospheric thermal inversion layer was observed in both the night and day. It developed near 60 km in altitude, progressed downward at 0.40±0.06 km/h, and had an overlying adiabatic lapse rate. The inversion amplitude correlated with the evolution of stratospheric gravity wave activity, although the mesospheric perturbations were too large to be due to conservative gravity-wave growth alone. The 24 h mean temperature profile shows no evidence of a residual inversion layer.