OGO for magnetic field observations in high beta regions of magnetospher

Heppner, J. P.

Ledley, B. G.

Skillman, T. L.

Sugiura, M.

English

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	 I 'k A 3 & yS
MAGNETIC FIELD OBSERVATIONS
IN HIGH BETA REGIONS
OF THE MAGNETOSPHERE
1
M. SUGIVRA
T.	 L. SKILLMAN
B. G. LEDLEY
J. P. HEPNER ^^2 141S1o'>>^
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AUGUST 1969
GODDARD SPACE FLIGHT CENTER
GREENBELT, MARYLAND
a N &g 3 6 0 5 4
£	 (ACCESSlO Ur^MBER)	 l4IRU)0Y (PAGES)
	 i 	 (CODE)
{NASA C OR T X OR AD NUMBER)
	 {CA	 ORY)
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MAGNE'L'C FIELD OBSERVATIONS IN HIGH $ RLGIONS
OF THE MAGNETOSPHERE
M. Sugiura
T. L. Skillman
B. G. Ledley
and
J. P. Heppner
August 1969
Presented at the Summer Advanced Study Institute, "Earth's Particles
any : Fields,, 1969", Santa Barbara, California, August 4-15, 1969.
 
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A 2ROXIMATE DISTRIBUTION OF AB
For a description of the distortions of the geomagnetic field in the
magnetosphere, AB provides a useful measure. Figure 1 shows approximate
equi-AB contours for the two 90 0
 sectors centered at the noon and mid-
night meridian half-planes, based on a preliminary sampling of a much
greater body of OGO 3 and 5 Rubidium magnetometer data
For this preliminary presentation the data for the southern hemisphere
were combined with those for the northern hemisphere assuming symmetry
with respect to the geomagnetic equator in spite of the fact that the tail
neutral sheet does not necessarily coincide with the geomagnetic equator.
The data used in Figure 1 were taken during periods when Kp was 0 or 1.
The negative AB values in the equatorial region between L = 2 and 6
represent the field depression from the quiet time ring current. The
maximum field decrease is approximately 35y. The AB > 0 regions at higher
latitudes on the nightside reflect the combined effects of the magnetic
field inflation due to the equatorial ring current, the neutral sheet
current in the tail, and the boundary surface current. There is an indi-
cation that a region of slightly low AB exists at about 20 to 30 0 latitude
and between 6 and 8 Re, but it is not certain whether this is real or
from a seasonal effect that is not taken into account in averaging. It
is noted, however, that there is a similar tendency in the AB contour map
for Kp = 2 or 3 to be shown in Figure 2.
On the front side, th y: AB = 0 line drawn at low latitudes with the
present data matches very well the corresponding line at higher latitudes
determined by Heppner et al. (1967) from the OGO 1 results. The change
of the sign for AB across this line is mainly from the boundary surface
current.
11	 1
rp:.
r- 2 -
An equi-AB contour map based on the data taken during slightly dis-
turbed periods with Kp = 2 or 3 is presented in Figure 2. This AB map
differs from that shown in Figure 1 for magnetically quiet periods in
several respects: (i) the ring current is intensified; (ii) the tail field
is increased; (iii) AB in the high AB region (above — 30 0 latitude) becomes
larger; and (iv) the AB = 0 line on the front side changes its shape.
Parallelism between Dst (mainly representing the ring current field)
and are (a measure for polar substorm activity) has been noted earlier
(Sugiura, 064), but a comparison of Figures 1 and 2 clearly demonstrates
that even during relatively weak substorms the quiet time ring current
x.B intensified. Increases in the tail field during high Kp periods have
1
been discussed by Benannon and Ness (1966) and Ness and Williams (1966).
Calculations based on the boundary sur"ace current model of Mead (1964)
and the tail current model of Williams and Mead (1965) show that the high
AB values exceeding 40-y or even 50Y observed by OGO 1, 3, and 5 above
300 latitude cannot be explained by these models, confirming the earlier
conclusion reached by Heppner et al. (1967) from the OGO 1 observations.
Still unanswered is the question of whether the magnetic field inflation
aused by the ring current is enough to account for such high AB values,
and if not enough, whether there is an additional pressure exerted from
the polar regior of the magnetosphere. In a later, more comprehensive
analysis the possibility that the AB distribution depends on not only
instantaneous but also previous magnetic activity will be examined.
	 ,
- 3 -
SPATIAL DISCONTINUITIES IN THE NEAR TAIL REGION
The high A region in the nightside magnetosphere is referred to here
as the %ear tail" region. The dynamics of this region appears to be a
key to the understanding of magnetospheric disturbance phenomena. In
this section, magnetic field structures in the near tail region under
relatively quiet conditions are discussed.
Figure 3 shows the variation in B on an inbound pass of OGO 3 near
midnight. The steady tail field is abruptly interrupted by a sudden de-
crease in B at 11.3 Re and at 12.7 0 geomagnetic latitude. Such a sudden
decrease in B followed by irregular field variations is a feature repeatedly
observed by OGO 3 on its nightside low latitude passes. When the satellite
crosses the geomagnetic equator near 10 Re the sudden drop in B is dramatic
as seen in the example shown` in Figure 4. In contrast to the sharpness
of the variation in its magnitude the direction of the magnetic field
does not generally indicate any abrupt change.
By estimating the kinetic energy density of the plasma based on the
electron flux measurement by Frank (1967,
  1968a, 1968b) it is found that
the plasma kinetic energy density abruptly increases at the time a share
decrease is observed in B. Plasma measurements in the tail by the Vela
group indicate that the proton kinetic energy density is greater than
(and probably several times) the electron kinetic energy density (Barre
et al., 1967). If this is so then it can be concluded that the abrupt
decrease in B corresponds to a traversal of a discontinuity between a
<< 1 region and a a > 1 region. Sometimes two or more step type dis-
continuities are observed on one pass. Thus the near tail region appears
to have shell like structures with a-discontinuity between successive
shells filled with plasmas of different kinetic energy densities.
ON
w.
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- 4 -
TAIL FIELD CHANGES DURING SUBSTORMS
The OGO 1 and 3 magnetic field measurements in the tail indicated
that in association with polar substorms on the earth, magnetic field
disturbances are observed in the tail only near the meridian plane of
the negative bay onset on the earth, that the tail field changes are
delayed relative to the bay onset time on the ground by several to
fifteen minutes, and that the tail field changes are such as to approach
the dipolar configuration (Heppner et al., 1967; Sugiura et al., 1968).
Thus at the onset of a negative bay the plasma pressure is abruptly
released in the near tail region in the vicinity of the magnetic mid-
night meridian because of a highly channeled, rapid convection of the
plasma (Heppner, 1969) or a drainage of the plasma triggered by the bay
onset, and the tail magnetic field collapses beyond this region. This
collapsing process proceeds tailward in a relatively narrow region
along the magnetic midnight meridian. The electron behavior in the
tail plasma sheet observed by Vela 2 during magnetic bays (Hones et al.,
1967) seems to support this picture.
HIGH p REGION ON THE DAWN FLANK OF THE MAGNETOSPHERE
The OGO 1 magnetic field measurements have shown (i) that at low
geomagnetic latitudes near the dawn meridian the average gradient in B
is essentially zero between — 11 Re and the magnetopause; (ii) that
frequently there is very little contrast between magnetospheric and mag-
netosheath fields near the magnetopause, making boundary identification
difficult using the magnetic field data alone; and (iii) B in the magneto-
sheath is sometimes greater than B immediately inside the magnetopause
e- 5 -
(Heppner et al., 1967). Based on these observations it was concluded
that ^ must be approximately equal to, or greater than, unity in the low
field-gradient region near the dawn meridian.
4
The more recent OGO 5 observations have confirmed the OGO 1 results,
and have provided examples in which this equatorial high P region extends
to near local time 9 hours. In addition, whereas the previous argument
for the presence of high a plasma in the region concerned was :solely
based on the magnetic field observations, the OGO 5 electric field mea-
surements show that the total plasma flux in this equatorial region is often
comparable to, if not greater than, the plasma flux in the adjacent mag-
netosheath.	 r 1
SUMMARY
The following aspects of the distortion of the geomagnetic field in
the magnetosphere are discussed on the basis of the magnetic observations
by the OGO 1, 3, and 5 satellites:
(1) Approximate distributions of AB (the magnitude of the measured
field minus the magnitude of the reference field) are given in the noon
and midnight sectors of the magnetosphere for magnetically quiet (Kp = 0
or 1) and slightly disturbed (Kp = 2 or 3) conditions. The field depression
in the region of L < 6 caused by the quiet time ring current is shown to
increase even during weak disturbances. The existence of regions of large
AB above about 300
 geomagnetic latitude cannot be accounted for by the
existing theoretical models for the magnetopause surface current and the 	 x
neutral sheet current of the tail, and is unlikely to-be explainable by 	 F
Pr	
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- 6 -
the inflation due to the ring current. Large AB values in these regions
may be a result of an additional pressure exerted from the polar regions
of the magnetosphere.
(2) A sudden decrease in the magnetic field followed by irregular
variations was often observed near midnight on low latitude inbound orbits
of OGO 3. Comparing with the plasma observations this sudden decrease is
interpreted as a traversal of the boundary between a low P region of the
tail and an equatorial region of 0 > 1 (i.e., the plasma sheet).
(3) Following the onset of a negative bay disrurban -e on the ground
the magnetic field in the tail collapses near the meridional plane of the
. bay onset. Thus the bay -associated tail field change is such as co make
the field configuration more dipolar.
( ) On the dawn side of the magnetosphere the magnetic field gradient
is near zero in the equatorial region between about 11 Re and the magneto-
pause. In this region the magnetic field in the magnetosheath is often
comparable to, or even greater than, the field immediately inside the
boundary. These observations suggest presence of a high 0 plasma in this
re gion .
f1
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REFERENCES
Bame, S. J., J. R. Asbridge, H. E. Felthauser, E. W. Hodes and J. B.
Strong, J. Geophys. Res. 72, 113, 1967.
Behannon, K. W., and N. F. Ness, J. Geophys. tics. 71, 2327, 1966.
Frank, L. A., J. Geophys. Res. 72, 185, 1967.
Frank, L. A., Earths Particles and Fields, (ed. by B. M. McCormac),
Reinhold, New York, 67, 1968x.
Frank, L. A., Physics of the Magnetosphere, (ed. by R. L. Carovillano,
J, F. McClay, and H. R. Radoski, D. Reidel, Dordrecht-Holland, 271,
1968b.
Heppner, J. P., Atmospheric Emissions, (ed. by B. M. McCormac), Reinhold,
New York, 1969.
Heppner, J. P., M. Sugiura, T. L. Skillman, B. G. Ledley, and M. Campbell,
J. Geophys. Res. 72, 5417, 1967.
Hones, E. W., Jr., J'. R. Asbridge, S. J. Bame, and I. B. Strong, J. Geophys.
Res. 72, 5879, 1967.
Mead, G. D., J. Geophys. Res. 69, 1181, 1964.
Ness, N. F. and D. J. Williams, J. Geophys. Res. 71, 322 ) 1966.
Sugiura, M., A.Lin IGY 35, 9, 1964.
Sugiura, M., T. L. Skillman, B. G. Ledley, and J. P. Heppner, J. Geophys.
Res. 73, 6699, 1968.
Williams, D. j. and G. D. Mead, J. Geophys. Res. 70, 3017, 1965.
FIGURES
Figure 1. Equi-AB contours for 900 sectors centered at the noon
and midnight meridian half-planes, in geomagnetic dipole
coordinates; Kp = 0 and 1.
Figure 2. Equi-AB contours for 900
 sectors centered at the noon and
midnight meridian half-planes, in geomagnetic dipole co-
ordinates; Kp = 2 and 3.
Figure 3. B in the low latitude near tail region. Kp, L, local time,
geomagnetic dipole latitude, and geocentric distance in
earth-radii are indicated on the top.	 f
Figure 4. B in the equatorial near %.ail region.
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Magnetic field observations in high beta regions of the magnetosphere

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