Ionospheric Response to Traveling Convection Twin Vortices

GEOPHYSICAL RESEARCH LETTERS, VOL. 21, NO. 17, PAGES 1759-1762, AUGUST 15,1994

JonoS heric response to traveling convection twin vortices

It W. Schunk, L. Zhu, and J. J. Sojka Center for Atmo pheric and Space Sciences, Utah State University, Logan, Utah 84322-4405

Abstract. Traveling convection twin vortices have been move in the antisunward direction for a period of several hours, bserved for everal years. At ionospheric altitudes, the twin with about a 15-minute time interval between vortices. ~ortiCes correspond to spatially localized, transient structures Mounting evidence indicates that the twin vortices occur on embedded in large-scale background convection pattern. The closed magnetic field lines and most likely those field lines convection vortices are typically observed in the morning and that connect to the low-latitude boundary layers (LLBL) evening regions. They are aligned predominantly in the east [Heikkila et ai., 1989; McHenry et ai., 1990a]. At present, the west direction and have a horizontal extent of from 500-1000 prevailing view is that the twin vortices are not a result of flux km. Associated with the twin vortices are enhanced electric transfer events at the dayside magnetopause, but their exact fields, particle precipitation, and an upward/downward field cause is still controversial. Friis-Christensen et ai. [1988] aligned cu rrent pair. Once formed, the twin vortex structures suggested that the traveling vortices are due to sudden changes propagate in the tail ward direction at speeds of several km/s, in the solar wind dynamic pressure or the interplanetary but they weaken as they propagate and only last for about 10- magnetic field (lMF). Heikkila et al. [1989] suggested that the 20 minutes. Because these convection structures might have a twin vortices are due to penetrating solar wind plasma clouds, significant effect on the localized ionosphere, the USU while McHenry et ai. [1990b] proposed that the vortices are ionospheri c model was used to calculate the response of the due to a K-H instability at the LLBL. Unfortunately, to date, ionosphere to "representative" traveling convection twin very little work has been done with regard to quantitative vortices for a range of background conditions. The electrodynamic modeling of traveling convection twin ionospheric response includes localized temperature vortices, although conceptual models of vortex generation enhancements, ion composition changes, non-Maxwellian have been developed that are based on the transient response ion distributi ons, and plasma upwelling events. The response of the magnetosphere to sudden solar wind dynamic pressure is transient and the magnitude of the response depends on the changes [Sibeck, 1990; Kivelson and Southward, 1991]. background ionospheric conditions and on the characteristics Our interest is not in modeling the traveling vortex of the twi n vortices. structure, but in calculating the ionospheric response to such a structure. Therefore, any electrodynamic model that produces self-consistent electric field and particle precipitation patterns 1. Introduction that are consistent with the measurements is adequate for our purposes. To this end, we used the electrodynamic model Ground- ba ed magnetometers have observed the signatures developed by Zhu et al. [1993] to obtain a "representative" of traveling convection twin vortices [Friis-Christensen et ai., traveling convection twin vortex. The ionospheric response 1988; Heikkila et ai., 1989; McHenry et al., 1990a, b; to this vortex structure was then calculated with the USU KiveLson and Southward, 1991; Glassmeier and Heppner, ionospheric model [Schunk, 1988; Sojka, 1989]. 1992]. The twin vortices correspond to spatially localized, tranSient, convection cells embedded in a large-scale convection pattern. The vortices are characterized by 2. Ionospheric Model enhanced electric fields, particle precipitation, and an The USU time-dependent ionospheric model (TDIM) is a UPward/do nward field-aligned current pair. They are typically mUlti-species (NO+, N;, A+, N+, He+) model that is based observed in the prenoon and postnoon sectors at geomagnetic 0;, on a numerical solution of the coupled continuity, momentum, latitudes between 60-75" and are aligned predominantly in the and energy equations. The TDIM is a Lagrange-Euler hybrid east-west direction. The E-W extent of a vortex structure model in that the equations are solved as a function of altitude ranges fro m everal 100's to 1000 km and its N-S extent is on a fixed grid for horizontally convecting plasma flux tubes. about 500 km. Although the field-aligned currents are The 3-dimensional nature of the model is obtained by ~POSite i.n a given twin vortex, either polarity is possible. following numerous flux tubes in a given simulation. e magOl tude of the current is estimated to be about I IlA/m2 However, the TDIM requires several global inputs, with the and the characteristic energy of the precipitation in the upward primary ones being the atmospheric parameters and the current cell i about 1 keY. Once formed, the twin vortices magnetospheric convection and precipitation patterns. ;~opagate toward the nightside at speeds of from 3-6 krnls, but The MSIS atmospheric model was adopted for the ~y weaken as they propagate and only last for about 10-20 simulations, and both summer (day 173) and winter (day 357) :IOUtes. Although single vortex structures are more common, cases were considered for low magnetic activity (Ap 9, Kp COntinuo us eries of traveling twin vortices was observed to = = 2+) and moderate solar activity (F1O.7 = 130). The neutral wind Cop . was described by our previously used simple prescription with Ynght 1994 by the American Geophysical Union. a 100 mls N-S component. With regard to the magnetospheric Pape inputs, both "background" and "twin vortex" convection and 00 r number 94GLO 1059 precipitation patterns were needed. Our selection of the 4 9 -853 4/9 4/94GL_0 1059$03 .00 background patterns was guided by observations, which 1759 1760 SCHUNK ET AL.: IONOSPHERIC RESPONSE TO TRAVELING CONVECTION TWIN VORTICES

UT 12:01 indicated that twin vortices favor positive IMF Bz conditions UT 12:03 UT 12:06 and relatively quiet geomagnetic activity [McHenry et ai., 1990b]. Therefore, the background patterns we adopted were the Heppner-Maynard "distorted DE" convection pattern for northward Bz [Heppner and Maynard, 1987] and the Hardy et ai. [1985] auroral oval for Kp = 2+. Note that different background convection and precipitation patterns could have been adopted, which would yield different background ionospheric conditions. However, the ionospheric response to a traveling twin vortex would be qualitatively similar. The convection and precipitation signatures associated with the traveling twin vortex were obtained from the Zhu et ai. [1993] electrodynamic model using several simplifying assumptions. First, it was assumed that a moving field-aligned current pair was generated owing to the interaction of the solar wind with the LLBL. The current filaments were initiated in the MIN .·12.6 MAX . 14.8 MIN . · 10.6 MAX . 13.1 dawn sector, and then they were assumed to move toward the MIN . ·7.4 MAX. 8.3 nightside at a speed of 3 krn/s. A Gaussian was assumed to Figure 2. Snapshots of the potential distribution associated represent the current distribution in each filament, with a with the twin vortex. The contour spacing is 3 kY. The maximum current of 1.2 ~A/m 2 for both the upward and maximum and minimum potentials are 14.8 and -12.6 kV downward current filaments initially. As the twin vortex (left), 13.1 and -10.6 kY (middle), and 8.3 and -7.4 kV (right). moved toward the nightside, the currents were assumed to decay in time. The field-aligned current filaments were sustained by a downward propagating Alfven wave and, coupled with an toward the nightside at a speed of 3 km/s, its intensity ionospheric boundary condition, this allowed us to calculate decreases and it disappears after approximately 15 minutes. self-consistent convection and precipitation patterns for a The nightside and dayside cells also correspond to downward traveling twin vortex. However, for the calculation of the and upward field-aligned currents, respecti vely. Precipitating precipitation pattern, we made the additional assumption that electrons only occur in association with the upward current only' the upward current filament was associated with (dayside cell), as explained earlier (not shown). Initially, the precipitating electrons. The resulting energy flux distribution maximum precipitating energy flux is 2.3 ergs cm·2 S· I. was calculated according to Fridman and Lemaire [1980] . Note that the "representative" traveling twin vortex shown Before presenting the vortex patterns, it is useful to in Figure 2 has features that are both qualitatively and introduce the high-resolution spatial grid used in our quantitatively in agreement with the observations. Therefore, simulations (Figure I). This grid is located in the dawn sector these self-consistent inputs can be used in the TDIM to of the polar region and is large enough to contain the traveling calculate the ionospheric response to a traveling twin vortex. vortex during its lifetime. Snapshots of the electrostatic potential distribution that is associated with the traveling twin vortex are shown in Figure 2. The vortex was initiated at 12 3. Ionospheric Simulation UT in the high-resolution grid on the dayside. The potential The USU TDIM was used to calculate the ionospheric distribution represents twin convection cells with the plasma response to the traveling twin vortex. The ionospheriC convection counterclockwise in the nightside cell and parameters were obtained at altitudes from 90 to 800 km with a clockwise in the dayside cell. Initially, the maximum 4 km spatial step. Approximately 1000 plasma flux tubes convection electric field is about 100 mY1m (initial circulation were followed, with the result that one flux tube always existed speeds of about 2 krnls), but as the twin vortex structure moves in each of the grid cells shown in Figure 1 at every time step. This yielded a 1 km horizontal spatial resolution. The plasma flux tubes were interpolated to the grid locations to obtain the 1200 MLT magnetospheric inputs associated with the twin vortex. The time step used in the calculation was typically 5-10 sec for a flux tube when outside the twin vortex and I sec for when it is inside the vortex. Initially, a diurnally reproducible ionosphere was calculated using the background convection and precipitation patterns. Then, at 12 UT, the ionosphere

1800 0600 was subjected to the additional convection and precipitation inputs associated with the twin vortex. Because the speed of 80 the vortex structure was 3 km/s , individual plasma flux tubeS were directly affected by the vortex for only a brief period of 70 time. 60 Our calculations indicated that the twin vortex had its greatest effect on the ionosphere at altitudes between about 0000 140-300 km, with the maximum effect occurring near 200 kIn. The effect was also the largest during the first 6-8 minutes af: Figure 1. High-resolution spatial grid used in the the appearance of the vortex. In response to the enhanc simulations. The grid is shown in an MLT-magnetic latitude electric fields and particle precipitation associated :,~th =: coordinate system and is approximately 2000 x 4000 km. twin vortex, several ionospheric processes were anttclpat , SCHUNK ET AL.: IONOSPHERIC RESPONSE TO TRAVELING CONVECTION TWIN VORTICES 1761

UT 12:01 UT 12:03 UT 12:06 UT 12:01 UT 12:03 UT 12:06

4.2 4.4 4.6 4.8 5.0 5.2 4.4 4.5 4.6 4.7 4.8 4.9 5.0 3 Log ,o cm·3 Log,o cm·

Ti Te

I _ ~ ] o 1000 2000 3000 4000 1800 1900. 2000 -2100 2200 2300 OK OK PJaiC J

Plate 1. Snapshots of the 0+ and NO+ densities, and the ion and electron temperatures, at 220 km. The si mulation results are for moderate solar activity and summer conditions.

included T; and Tt enhancements, ion temperature anisotropies, coupling to the hot ions and energy transfer from the and 0 + ~ NO+ composition changes [cf. Schunk et al., 1975; precipitating electrons. The latter mechanism dominates, and St.- Maurice and Schunk, 1979]. However, since the twin since it only exists in the dayside cell (upward current region), vo rtex moves fairly rapidly through the ionosphere and the the TI' enhancement is asymmetric (see 12:01 UT). different ionospheric parameters have different response As the twin vortex moves through the ionosphere, it decays ti mes, the quantitative effect of the traveling twin vortex can and the ionosphere starts to recover after its passage. only be determined via a numerical simulation. Nevertheless, the residual effect of the vortex motion can be

Plate 1 shows snapshots of n(O+), n(NO+), Tj and TI' at 220 clearly seen as streaks in the n(O+), n(NO+) and T I' km and at 12:01 , 12:03 , and 12:06 UT for summer conditions. distributions, but not in the T j distribution. Because of the These parameters are only shown in the grid area of Figure 1. short chemical time constants at 220 km, the density streaks In response to the enhanced vortex electric fields, T; is are not uniform, with the largest depletions/enhancements initially increased to about 4000 K in both convection cells occurring at the present location of the twin vortex. Because due to ion-neutral frictional heating. Associated with the of the rapid T; response time, it is only elevated at the present increased T;'s is a substantial 0 + ~ NO+ composition change location of the twin vortex. Tt also has a rapid response time Ow ing to the energy dependence of this reaction. Initially, to external inputs. However, a residual streak in its n(O+) is decreased by a factor of 10 and n(NO+) is increased by a distribution occurs because of the inverse relationship between fac tor of 2.5. Like T;, Tt is also elevated in the twin vortex T, and Ne. An Ne depletion in the vortex wake yields a T t locati on, but only by about 300 K. Unlike T;, however, the TI' increase in response to the fairly uniform "background" enhancement is not symmetric. Tl' is increased due to both convection and precipitation inputs.

~ ~------~---r------~/ --~r-Tf'------~--~ 700 J

GOO /' I

500 /' I ! I ~ 4 oo i :"l 300 i Ti l 200 .// 100

o ~------~ __ --__ ---4----__-- __----~--~-- __---- __------~ I 3 4 1000 2000 3000 1000 2000 3000 ' 000 Log,. o.nslly (cm4) Tempertlurt (K) Ttmpertluro (K)

Figure 3. 0 + and NO+ densities (left) and Te and T; (middle) versus altitude for locations inside (solid curves) and outside (dashed curves) the twin vortex at 12:03 UT. The right panel shows Till and T;.l versus altitude for the location in the twin vortex. 1762 SCHUNK ET AL.: IONOSPHERIC RESPONSE TO TRAVELING CONVECTION TWIN VORTICES

The effect of the twin vortex as a function of altitude is that the beam positioning technique used was not optimum for shown in Figure 3, where profiles of n(O+), n(NO+), T; and Te measuring traveling vortices and only line-of-sight velocities

are given for locations both inside and outside the downward and T; data were presented at only four altitudes (no Te, Ne Or current region of the vortex at 12:03 UT. For T;, the largest ion composition data were presented). The measured effect occurs at low altitudes where frictional heating velocities, elevated T;'s and non-Maxwellian distributions are dominates, while for TI' it occurs at high altitudes and is due to consistent with our calculations. heating from the hot ions and the decrease in Nc' For 0 +, the largest effect is in the E and lower F regions where the 0 + + N2 Acknowledgments. This research was supported by NASA grant -7 NO+ + N reaction dominates, while for NO+ the increased NAG5-1484 and NSF grant ATM-93-08163 to Utah State University. scale height associated with the elevated temperatures yields We thank Mike Bowline for computational assistance. large enhancements at all altitudes above 200 km. The parallel and perpendicular ion temperatures that are References associated with the vortex T; profile are also shown in Figure Fridman, M., and J. Lemaire, Relationship between auroral electrons 3. The anisotropy, with Til. > T;II ' is largest at low altitudes fluxes and field-aligned potential difference, 1. Geophys. Res., 85, where the ion-neutral frictional heating is largest. The 664, 1980. decrease in the anisotropy with altitude is primarily due to the Friis-Christensen, E., M. A. McHenry, C. R. Cl auer, and S. Vennerstrom, isotropic scattering associated with Coulomb collisions. Ionospheric traveling convection vortices observed near the polar As noted above, the electron precipitation only occurs in cIeft: A triggered response to sudden changes in the solar wind, the upward current region. Its main effect is to increase TI!' Geophys. Res. Lett., 15, 253, 1988. The increase in Nc associated with the precipitation is only Glassmeier, K., and C. Heppner, Traveling magnetospheric convection about 25% in the E-region and is negligible in the F-region. twin vortices: Another case study, global characteristics and a Hence, in general, the vortex electric field has a much larger model, 1. Geophys. Res., 97, 3977-3992, 1992. Hardy, D. A., M. S. Gussenhoven, and E. Holeman, A statistical model effect on the ionosphere than the vortex precipitation. of auroral electron precipitation, 1. Geophys. Res., 90, 4229-4248, The increased ion scale heights shown in Figure 3 are a 1985. manifestation of plasma upswelling in the twin vortex in Heikkila, W. J., T. S. Jorgensen, L. J. Lanzerotti, and C. G. Maclennan, response to the elevated TI' and Ti. This could also lead to an A transient auroral event on the days ide, 1. Geophys. Res .. 94, 15291, enhanced polar wind, but the simulation of that flow is beyond 1989. the scope of this paper. Heppner, J. P. and N. C. Maynard, Empirical high-latitude electric field models, 1. Geophys. Res., 92, 4467-4489, 1987. Kivelson, M. G., and D. J. Southward, Ionospheric traveling vortex 4. Summary generation by solar wind bu ffeting of the magnetosphere, J. The transient ionospheric response to a "representative" Geophys. Res., 96, 1661-1667, 1991 . traveling twin vortex was calculated for moderate solar activity Lockwood, M., B. J. I. Bromage, R, B. Horne, l oP. St.-Maurice, D. W. Willis, and S. W . H. Cowley, Non-Maxwellian ion velocity and both summer and winter conditions. The ionospheric distributions observed using EISCAT, Geophys. Res. Lett., 14, 111- response was characterized by ion and electron temperature 114, 1987. enhancements, ion temperature anisotropies, elevated plasma Liihr, H., W. Blawert and H. Todd, The ionospheric plasma flow and scale heights, 0 + density depletions, and NO+ density current patterns of traveling convection vortices: A case study, J. increases. In general, the effects were the largest in the E and Almos. Terr. Phys., 55, 1717-1727, 1993. lower F regions, although the effects were evident at all McHenry, M. A., C. R. Clauer, E. Friis-Christensen, P. T. Newell, and J. altitudes. The effects in winter (not shown) were comparable D. Kelly, Ground observations of magnetospheric boundary layer to, but smaller than, the summer effects shown in this paper. phenomena, 1. Geophys. Res .. 95, 14995. I 990a. The distinct ionospheric signatures associated with a twin McHenry, M. A., C. R. Clauer, and E. Friis-Christensen, Relationship of vortex may be useful in elucidating the magnetospheric olar wind parameters to continuous dayside, high latitude traveling iono pheric convection vortices, 1. Geophys. Res., 95, 15007, I 990b. ionospheric coupling processes associated with traveling twin Sibeck, D. G., A model for the transient magnetospheric response to vortices. sudden solar wind dynamic pressure variations, 1. Geophys. Res., 95, The ionospheric response we predict is large, but the effect 3755, 1990. is transient and may be difficult to measure. To date, by far the Schunk, R. W., A mathematical model of the middle and high latitude bulk of the twin vortex measurements have been made with ionosphere, PAGEOPH. 127, 255-303, 1988. ground-based magnetometers. With reasonable assumptions, Schunk, R. W., W. J. Raitt, and P. M. Banks, Effect of electric fields on such data have been used to deduce the speed of the vortex the daytime high-latitude E and F regions, 1. Geop/zys. Res .. 80, 3121 , structure and to estimate the field-aligned current magnitudes. 1975. However, measurements of the plasma densities and Sojka, J. J., Global scale, physical models of the F region ionosphere, temperatures inside the vortex require incoherent scatter radars. Rev. Geophys .. 27, 371-403, 1989. Unfortunately, our calculations indicate that the interpretation St.-Maurice, J.-P. , and R. W. Schunk, Ion velocity distributions in the high latitude ionosphere, Re v. Geophys .• 17, 99- 134, 1979. of the radar data will require a careful analysis. We predict both Zhu, L. , J. J. Sojka, R. W. Schunk, and D. J. Crain, A time-dependent rapid O+/NO+ composition changes and non-Maxwellian ion model of polar cap arcs, 1. Geophys. Res., 98, 6139, 1993. velocity distributions in the vortex at E and F regions. Both of these effects complicate the interpretation of the radar data. Nevertheless, EISCAT radar measurements pertaining to one R. Schunk, J. Sojka, and L. Zhu, Center for Atmospheric and spa~ . traveling vortex event have been made [Lockwood et al., Sciences, Utah State University, Logan, UT 84322-4405 (e-mail. 1987; Liihr et ai., 1993]. In the first paper, evidence of non [email protected]) Maxwellian ion distributions above 300 km was presented at the times when the ion drift speed in the vortex exceeded the (Received: November 23, 1993; revised: March 15 , 1994; accepted: neutral thermal speed. In the second paper, the authors noted April 13, 1994.)