As shown by ground-based absorption measurements, the lower ionospheric plasma is markedly controlled by the structure of the IMF. Whereas in high auroral and subauroral latitudes this effect is very pronounced, in midlatitudes its influence is less important. A comparison of these results with satellite data of the IMF and the solar wind speed confirms the important role of these components, not only during special events but also for the normal state of the ionospheric D region plasma

Bremer, J.

English

NASA Technical Reports Server

196 N90-28200
THE INFLUENCE OF IMF ON THE LOWER IONOSPHERE PLASMA
IN HIGH AND MIDDLE LATITUDES
J. Bremer
Academy of Sciences of the GDR
Heinrich-Hertz-Institute of Atmospheric Research and Geomagnetism
Observatory of Atmospheric Research
DDR-2565 Kiihlungsborn, GDR
Abstract: As shown by ground-based absorption measurements, the lower ionospheric plasma is
markedly controlled by the structure of the IMF. Whereas in high auroral and subauroral latitudes
this effect is very pronounced, in midlatitudes its influence is less important. A comparison of
these results with satellite data of the IMF and the solar wind speed confirms the important role of
these components, not only during special events but also for the normal state of the ionospheric
D-region plasma.
1. Introduction
As known from DUNGEY (1961) the vertical component B z of the IMF in the solar-
magnetospheric coordinate system markedly controls the energy transfer from the solar wind into
the magnetosphere. Whereas negative B z components should favor this transfer, positive B z
values should inhibit such an input.
Assuming an IMF which is emitted together with the solar wind parallel to the solar
equator, the B z component shows systematic variations due to daily and seasonal changes of the
position of the geomagnetic dipole axis of the Earth with respect to the solar rotation axis. In
Figure 1 the seasonal (a) as well as diurnal (b) variations of B z are presented for an IMF with A-
(full curves) as well as T- (dashed curves) polarity. After Figure la an IMF with A-polarity
induces positive B z values during the spring half-year and negative B z values during autumn,
whereas an IMF with T-polarity causes reverse signs of Bz, respectively. Following a proposal by
LAUTER et al. (1978) IMF sectors with negative B z values are called "pro sectors" and sectors
with positive B z values "anti sectors". The diurnal B z variation (Figure lb) having a markedly
smaller amplitude than the seasonal component has minima near 11 UT for A- and near 23 UT for
T-polarity.
In this paper the influence of the IMF on the plasma of the lower ionosphere shall be
investigated mainly during IMF sector boundary crossings (SBC). The dates of these SBC are
taken from a special list of WILCOX (1982).
2. Results
As the ionospheric plasma markedly depends on the highly variable solar EUV radiation as
well as internal atmospheric processes, it is often difficult or even impossible to separate these
phenomena from effects caused by the IMF sector structure. On the other hand, the individual
SBC data are sometimes incorrect. Therefore, here mainly the results of statistical investigations
are shown using the method of superposed-epoch analysis with the fn-st day of the new IMF sector
as the key day (zero).
In Figure 2 the results of such an analysis are shown using noon absorption data at 3 MHz
measured at the GDR Antarctic Station "Georg Forster" (70.77°S; 11.83°E) during June 1976 and
November 1978. Whereas in the upper part (Figure 2a) all SBC are used independently of the
direction of the IMF, in the other parts the total number of SBC are divided into transitions with
respect to IMF polarity (T --_ A transitions: Figure 2b; A ---) T transitions: Figure 2c) and
transitions with respect to the Bz component of the IMF (anti _ pro, pro ---) anti sector transitions:
https://ntrs.nasa.gov/search.jsp?R=19900018884 2020-03-19T21:12:44+00:00Z
197
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Figure 1. Mean seasonal (a) and diurnal (b) variation of the B z component of the IMF in the solar-
magnetospheric coordinate system for A- (full curves) and %-polarities (dashed curves) with an
IMF amplitude B = 5 nT.
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Figure 2. Mean variation of ionospheric noon absorp-
tion in Georg Forster station (70.77°S, 11.83°E) at 3
MHz during IMF sector boundary crossings independent
of IM direction (a), for crossings from T--) A polarity (b),
for crossings from A ---) T polarity (c) and for crossings
from pro ---) anti (dashed curve) as well as anti --+ pro
sectors (full curve) (d), respectively.
198
Figure 2d). In Figure 2a-c only small variations of ionospheric absorption are to be seen, only the
minimum near day -1 seems to be typical. The slightly enhanced absorption at A polarity (Figure
2b and c) also observed by MANSUROV et al. (1976) seems to be connected with enhanced
panicle precipitation during noontime because due to the diurnal B z variation (see Figure lb) near
noon IMF sectors with A-polarity induce lower Bz values than T-sectors. More pronounced
variations of ionospheric absorption are observed, however, if IMF sectors are distinguished by
their B z component. In pro sectors the absorption is higher by about 10 - 12 dB than in anti
sectors. The transition between both sectors is very steep with absorption changes of about 4 - 5
dB from day -1 to day 0. As can be seen from Figure 2d, but also from similar investigations with
other ionospheric absorption data as well as different geomagnetic indices (BREMER and
LAUTER, 1984; BREMER, 1986), the transition from anti ---) pro sectors is more pronounced
than the transition from pro --* anti sectors. As the Bz component of the IMF plays an essential
role in the ionospheric plasma, in the following only pro _ anti and anti _ pro SBC will be used.
To investigate the IMF influence on the plasma of the lower ionosphere in dependence on
latitude we used the results of the Finnish riometer chain. In the left part of Figure 3 daily mean
values of the hourly maximum CNA data at 27.6 MHz are shov,,, ti,r the period between February
17 and March 2, 1981. After WILCOX (1982) a sector boundary crossing from A to T sector
occurs between February 23 and 24, marked by the dashed line. Whereas before the SBC which
is an anti sector the absorption is very small at all stations; during pro sector conditions after
February 24 the absorption is markedly enhanced. The excessive absorption is highest in high
geomagnetic latitudes (shown in brackets in Figure 3) and is smaller in lower latitudes but
nevertheless remarkable also in 57.6°N geom. latitude. The geomagnetic Ap index shown at the
top of the right part of Figure 3 is also enhanced after the sector boundary crossing. Beside these
ground-based data also interplanetary data are shown (KING, 1986) like solar wind speed and the
IMF B z component as well as the energy coupling function c introduced by PERREAULT and
AKASOFU (1978). Whereas the solar wind speed is characterized by a minimum before the
sector boundary crossing the Bz component changes its sign from plus to minus. To demonstrate
this change more clearly, on February 24 half-day mean values have been calculated for B z and e
instead of daily mean values estimated otherwise. In conclusion we can state that during pro sector
conditions the energy input from solar wind into the magnetosphere markedly increases, followed
by precipitation of high energetic particles into the middle atmosphere and enhanced CNA.
At the top of the left-hand part of Figure 3 also the IMF data derived from ground-based
measurements (Solar Geophysical Data, 1981) are shown which in this case do not agree with the
SBC data from WILCOX (1982) who took into consideration also satellite data. Such
uncertainties in SBC data estimated from ground-based measurements are not so critical if
statistical investigations like superposed-epoch analyses are made only, since about 85% of these
SBC data should be correct (SVALGAARD, 1976).
In Figure 4 the results of superposed-epoch analyses are shown using again daily mean
values calculated from hourly maximum CNA data at 27.6 MHz measured at 7 stations of the
Finnish riometer chain. The SBC are characterized by dashed lines. For pro ---) anti sector
transitions as well as anti ---) pro transitions we always observe higher absorption during pro sector
conditions rather than during anti sectors. The differences between both conditions become
smaller with decreasing latitude but can also be detected at Nurmij_irvi in 57.6 ° geom. latitude.
Generally the increases of absorption with anti _ pro transitions are steeper than for pro ---) anti
transitions, mainly to be seen above 60 ° geom. latitude, a fact already mentioned above.
In order to compare the superposed-epoch analyses of Figures 2 and 4 with interplanetary
data in Figure 5, the results of similar analyses are shown using daily mean values of the solar
wind speed, the B z component and the _ function during the period from 1975 to 1979 where a lot
199
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Figure 3. Variation of CNA at different geomagnetic latitudes, geomagnetic Ap index as well as
satellite data of solar wind speed, Bz component of the IMF and energy transfer function _ during
the period from February 17 until March 2, 1981. The IMF polarity data (A or T) are from Solar
Geophys. Data and the SBC (dashed lines) from WILCOX (1982).
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Figure 4. Mean variation of daily mean values derived
from hourly maximum CNA data for 1979 until 1982,
during IMF sector boundary crossing from pro _ anti
and anti ---) pro sector conditions in dependence on geom.
latitude. The dashed lines mark the SBC after WILCOX
(1982).
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Figure 5. Mean variation of the solar wind speed v, the
IMF B z component and the energy transfer function
calculated for transitions of IMF pro --_ anti and anti
pro sector conditions using satellite data 1975-1979.
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201
of such satellite data is available (KING, 1979, 1986). The mean variation of the solar wind speed
is very similar for pro --* anti and anti ---) pro sector transitions with a pronounced minimum at day
-1 just before the SBC. As to be expected, the Bz component is negative during pro sectors, but
positive during anti sectors. The energy coupling function E is higher during pro sectors than
during anti sectors. Whereas e shows a steep increase during the anti --) pro sector transition, the
decrease of e during pro _ anti transitions is not so steep and even shows a small secondary
maximum just after the SBC on day 0.
3. Discussion and Conclusions
As demonstrated by Figure 5, the energy transfer from solar wind into the magnetosphere
connected with a precipitation of high energetic particles into the middle atmosphere mainly
depends on the B z component of the IMF. Thus during times with negative B z values enhanced
fluxes of precipitating particles induce a higher ionization level in the ionospheric D region and
therefore higher absorption of radio waves as observed in Figures 2 - 4. Therefore, the proposed
classification of IMF sector in pro and anti sectors after their B z component seems to be well
justified.
Whereas the main differences between pro and anti sectors are caused by the B z component
also the solar wind speed v modulates the particle precipitation and, therefore, the ionospheric
absorption since _ is proportional to v. Thus the relatively flat variation of absorption during pro
--) anti sector transitions (Figures 2 and 4), connected with the secondary maximum of the solar
coupling function e (Figure 5), is caused by the steep increase of the solar wind speed at the front
of the anti sector. In geomagnetic indices even a secondary maximum can be observed during the
transition from pro ---) anti sector (BREMER, 1986). Also the absorption minimum on day -1 just
before the SBC in Figure 2a where all SBC were used irrespective of their polarization, is caused
by the minimum of solar wind speed (see Figure 5) as in this case the influence of the B z
component is covered by the averaging process.
As the precipitation of high energetic particles becomes smaller with decreasing geom.
latitude also the effect of IMF sector structure becomes less important in midlatitudes (Figures 3
and 4). This fact could be confirmed by investigations using LF absorption data at Central Europe
not shown here. Only when using a large amount of data, small effects similar to those discussed
here could be detected. At geom. latitudes above about 60 °, however, the IMF influence may be
essential if the concept of pro and anti sectors is used.
References
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auroralen und sub auroralen Breiten, Geod. Geoph. Verrff. R.I.H. 13, 3-14, 1986.
Bremer, J., and E. A. Lauter, Dependence of the high latitude middle atmosphere ionization on
structures in interplanetary space, Handbook for MAP 10, edited by J. Taubenheim, 200-204,
1984.
Dungey, J. W., Interplanetary magnetic field and the auroral zones, Phys. Rev. Lett., 6, 47-48,
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Ionospheric Absorption Data, Geophysical Observatory Sodankyl_i of the Finnish Academy of
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Space Flight Center, Greenbelt, MD 1979 and 1986.
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interplanetary magnetic field, Phys. Solariterr., Potsdam 7, 73-82, 1978.
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Mansurov, S. M., L. G. Mansurova, and Z. Ts. Rapoport, Ionospheric radio wave absorption in
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The influence of IMF on the lower ionosphere plasma in high and middle latitudes

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