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Andoya

In August and September 1967, two sounding rocket experiments, including barium releases, were carried out from the rocket range at Andoya, Norway. Flight 1 took place on August 31 and Flight 2 on September 2. Both salvos were launched into active conditions in evening twilight. Visual auroral structures were present in the observation region during both flights. Figure 3 shows that the electric field was much stronger during these experiments than in COPE 2, with magnitude fluctuations of up to 100 mV/m on spatial scales of less than 50 km and temporal scales less than 10 minutes. See Wescott et al. [1969] for details.

Joule heating effects of small-scale structure and neutral winds in the high-latitude thermosphere/ionosphere

L. Hurd and M. F. Larsen Department of Physics & Astronomy, Clemson University

Introduction

Barium clouds released from sounding rockets provide high-resolution time-varying measurements of the ion drift velocity within a spatially limited region, thus yielding accurate small-scale electric field information that may be lost when using other techniques, such as ground-based radar. Moreover, the resolution of most models is not sufficient to discern structure in the electric field at spatial scales less than a few degrees of latitude/longitude or time scales of 15 minutes or less. Since the Joule heating in the ionospheric E-region is proportional to the square of the electric field through the relationship

  fluctuations that may average to zero can still have a non-zero variance resulting in a significant contribution to the net heating rate. Here we present results from three barium chemical release rocket experiments in order to estimate the effects of this fine structure on the Joule heating in the polar latitudes for several different levels of magnetic activity. The primary analysis is from the COPE 2 campaign in March 1987 at Sondre Stromjord, Greenland, when six barium clouds were released in moderately disturbed conditions at dawn. A trimethyl aluminum (TMA) trail was also released to measure the E-region neutral winds, which also affects the heating profile. Further calculations based on results from two campaigns by Wescott et al. [1969] in Andoya, Norway, are also shown.

COPE 2

The COPE 2 sounding rocket campaign was carried out on March 21, 1987, with three rockets carrying barium, TMA, neodymium, and samarium launched within a 25 minute period into moderate (Kp < 4) conditions at dawn from the rocket range at Sondre Stromjord. Supporting radar and optical data show that discrete auroral arcs moved toward the launch site from the northeast starting at 6:30 UT. By the time of the observations between 6:58 and 7:15 UT, the arcs had reached the rocket range. The observed barium tracks and corresponding electric fields are shown in Figures 1 and 2. The neutral wind data is not shown (see Larsen et al. [1989]), but the wind field was shown to be relatively constant throughout the observation region. The wind vector completed a nearly full clockwise rotation through the lower E-region altitudes with magnitudes less than 100 m/s.

Email contact: Lucas Hurd <lhurd@clemson.edu>

Figure 1: Geographic map showing the barium cloud drift paths projected to a datum surface

at 100 km altitude for each of the six clouds. Numbers along the curves indicate minutes and seconds from 6:58:00 to 7:15:00 UT (see

Larsen [2003]).

Figure 2: (Top) Zonal and meridional components of the electric field calculated

from the Ba-2 cloud track from Fig 1. North and east are positive. The x-axis is MMSS after

7:00:00 UT. (Bottom) Electric field magnitudes calculated from three cloud tracks and the

average magnitude shown as the red dashed line.

Figure 3: (Top left) Barium cloud drift paths for Flight 1 projected to a datum surface at 100 km altitude. (Top right) Corresponding electric field values for the four tracks from Flight 1 and the average

magnitude shown as the red dashed line. The x-axis is MM after 22:08:00 UT. (Bottom left) Barium drift paths for Flight 2. (Bottom right) Flight 2 electric field values and average. X-axis is MM after 20:33:00 UT.

Figure 4: Height-resolved Joule heating profiles derived from all COPE 2 barium tracks plotted with the profile that results from averaging the electric field over the observation region (red).

The x-axis is 10-8 W/m3.

Figure 5: (black) Height-integrated Joule heating for I2 of Wescott Flight 1. (red) The

same calculation performed using the average electric field magnitude from Fig 3.

Figure 6: (Top left) The difference in the height-resolved Joule heating derived from the COPE 2 Ba-2 track including neutral wind effects and neglecting winds. The color bar is in units of 10-8 W/m3. (Bottom left)

The corresponding height-integrated Joule heating including winds (black) and neglecting winds (red). (Top right, Bottom right) Same calculations for the Ba-4 track, but note that the color bar is purely positive.

References Codrescu,  M.  V.,  T.  J.  Fuller-­‐Rowell,  J.  C.  Foster,  J.  M.  Holt,  and  S.  J.  Cariglia  (2000),  Electric  field  variability  associated  with  the  Millstone  Hill  electric  field  model,  J.  Geophys.  Res.,  105(A3),  5265.  Larsen,  M.  F.,  I.  S.  Mikkelsen,  J.  W.  Meriwether,  R.  Niciejewski,  and  K.  Vickery  (1989),  Simultaneous  observaTons  of  neutral  winds  and  electric  fields  at  spaced  locaTons  in  the  dawn  auroral  oval,  J.  Geophys.  Res.,  94,  17235-­‐17243.  Larsen,  M.  F.  (2003),  Greenland  COPE  2  chemical  release  experiments:  TMA,  barium,  neodymium,  samarium  results.  Internal  publicaTon.  Wesco-,  E.  M.,  J.  D.  Stolarik,  and  J.  P.  Heppner  (1969),  Electric  fields  in  the  vicinity  of  auroral  forms  from  the  moTons  of  barium  vapor  releases,  Released  NASA  manuscript  from  Goddard  Space  Flight  Center,  Accession  number  N69-­‐18687,  TMX-­‐63466.      

Acknowledgements The authors were partially supported by NSF grant AGS-1243467. MFL was also partially supported by NSF grant AGS-1007539. This work was supported by NASA Headquarters under the NASA Earth and Space Science Fellowship Program – Grant NNX13AM28H.

Electric field effects As shown in the above figures, there are significant small-scale fluctuations in the electric field vectors for all of the experiments. For COPE 2, note the changes in direction of the electric field along the Ba-2 track. This is probably due to the presence of subvisual auroral arcs present in the region north of the visual arcs. In Figure 4, we have plotted all of the height-resolved Joule heating altitude profiles calculated from the COPE 2 observations from Figure 1, along with the profile that would be calculated by using the region’s average electric field. To isolate the effects of the electric field, the electron density profile was taken to be the same for every location within the grid and the neutral winds were neglected. For several locations in space, the peak heating was underestimated or overestimated by up to a factor of 3. Figure 5 demonstrates that using the average electric field from Wescott’s first flight would result in a factor of 5 or more underestimation of the height-integrated heating in the region of the I2 cloud track.

Neutral wind effects The effect of the neutral winds on the Joule heating derives from the last 2 terms in the above equation. Note that the UxB term always serves to increase the heating but that the scalar triple product U!(ExB) may be either positive or negative depending on the direction of the ion velocity with respect to the neutral wind. Such effects were evident during COPE 2, as seen in Figure 6. Note that for the region traversed by the Ba-2 cloud, the neutral wind served to both increase and decrease the height-integrated heating depending on the electric field direction. Moreover, the location of the increase or decrease in heating is not uniform with altitude. It appears that the greatest effect is just below the peak in the Pedersen conductivity, with a second maximum/minimum just above the peak. Also note that for the region traversed by the Ba-4 cloud, the neutral wind always increased the heating. This suggests that even though the U!(ExB) changes sign, its magnitude is not enough to cancel the effect of the UxB term in this instance. Even though the effect of the neutral dynamics on the heating is not as great as the electric field effects, the winds still alter the height-integrated Joule heating by more than 12% at certain times and locations within the observation window, such as along the Ba-2 track.

Conclusions

Several ionized barium chemical release payloads have been analyzed for three sounding rocket campaigns. The results confirm that averaging the electric field over even a couple of degrees of latitude/longitude or over tens of minutes does not sufficiently capture the behavior of the field and the underlying physics. Previous theoretical studies (e.g., Condrescu et al. [2000]) have estimated the effect of sub-grid scale structure on the Joule heating to be as much as a factor of 2. This study suggests that the small-scale contribution can be even larger in active conditions. Our results also show that the neutral wind can have a non-negligible effect on the Joule heating and its altitude profile.

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