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Influence of interplanetary interaction regions on geomagnetic disturbances and tropospheric circulation
Phef. Spmr Ski., Vol 32, No. 12, pp. 1541-1545, Printed in Great Britain.
1984
0032LO633~84$3.00 + O.CfJ Per&mm Press Ltd.
INFLUENCE OF INTERPLANETARY INTERACTIO-N REGIONS ON GEOMAGNETIC DISTURBANCES AND TROPOSPHERIC CIRCULATION HENRIK LUNDSTEDT
Lund Observatory,
Box 1107, S-221 04 Lund, Sweden
(Received injnalform
30 April 1984)
Abstract--Proposed solar wind-ma~ctosphere energy coupling functions are studied. An empirical formula proposed by Svaigaard (1977) is found to predict the geomagnetic activity quite well. The influence of solar wind interaction regions on the tropospheric circulation, through a suggested cosmic ray mechanism, was investigated. The cosmic ray intensity at Earth clearly showed a decrease at the time of passage of an interaction region. It is suggested that the well-known dip in the Vorticity Area Index may be caused by an interaction-modulated decrease in cosmic ray intensity.
INTRODUCTION
parameters
It is well known that when a fast solar wind stream overtakes the trailing edge of a preceding, slow stream, a characteristic interaction region develops. The expression “interaction region” implies that within such a region fast and slow plasmas are interacting, by means of a transfer ofmomentum and energy, involving large amplitude waves and shocks. An interaction region is characterized by enhanced field magnitude (S), particle density (n) and temperature (7’) (Si&oe, 1972). These characteristics can be used to identify the interacting zones. Rosenberg and Coleman (1980) found 234 interaction regions, during the interval 1964 1973, using the enhanced interplanetary magnetic fieid, as a signal of an interaction region. Within the interaction region, there is a narrow zone called the interface, where the density drops sharply, proton tem~rature increases sharply, magnetic fields maximize and P = nkT+B2/8n peaks (Siscoe, 1972; Burlaga, 1974 ; Gosling et al., 1978). The gradient on the two sides of the interface results in stresses that are exerted both inward and outward. Theoretical studies of interaction regions have also been performed (Dryer et al., 1978). The physical processes involved in interaction region are now quite well understood. The Earth’s magnetosphere can be considered as a filter with the solar wind as the input and with the geomagnetic activity as output. Several different geomagnetic activity indices are presently used. The relationships between these indices and the power input are not clear. The index a, appears to increase linearly with power input. Svalgaard (1977) found the following relationship by studying the different solar wind
separately
1.157 x* a, = 6.6q(J; c() - 32 * K 21 105 (1 -i-3 cos.2 $)2’”
(l)
where nvg is the dynamic pressure, BI$ the influx of interplanetery magnetic field lines, TVis the angle between the interplanetary field and the geomagnetic field of the dayside magnetopause, f describes the variability of 8, the function q off and a is a fourth order polynom and $ is the angle between the solar wind flow direction and the dipole axis. It has been proposed that cosmic rays, modulated by variations ofsolar wind velocity and magnetic field may influence the tropospheric circulation. As a measure of the tropospheric circulation Roberts and Olson introduced the “vorticity area index” (VAI), a number that is high, when the area covered by relatively intense cyclonic circulation near 500 mb is large. Storminess is statistically correlated with high atmospheric vorticity. Wilcox et al. (1973) reported a sun--weather effect. They showed that the VAIt in the Northern Hemisphere during the winter period exhibits a pronounced dip a day or two after the boundary of an interplanetary sector sweeps past the Earth. Markson (1978) has proposed a mechanism, which involves the atmospheric electrical global circuit, to explain the observed sun-weather effect. In this theory current flows upward to the ionosphere from thunderstorms and then back through the fair-weather part of the atmosphere.
7 The VA1 should not be equated with “storminess” per se nor do Wilcox et al. claim this. Higher positive vorticity is however associated with disturbed weather conditions.
1541
H.
1542
LUNDSTEDT
Ionization above the thunderstorm area, and thus the magnitude of the atmospheric electric field, is changed due to variations in the cosmic ray intensity. Markson (1978, 1979) suggests that an intensified atmospheric electric field could cause more storminess and if so an enhanced VA1 would be expected. The cosmic ray intensity is increased after a solar flare and decreased after an interaction region passage. We thereforeexpect an increase of VA1 after a solar flare and a decrease of VA1 at times of interaction regions. Olson et al. (1975) reported a peak of VA1 on the first and second day of a flare. Markson’s work offers an explanation for how solar variability controls the electrification of the Earth’s atmosphere. It also proposes ways by which the changing Sun in turn may affect the weather.
where L, = L, = B: = V=
length of the magnetotail length of merging line merging component of magnetic field solar wind velocity.
In the so-called half wave rectifier model ST has been replaced by B, =O, B,=-BZ,
B, > 0 Bz
where B, is the North-South component of the interplanetary magnetic field. Perreault and Akasofu (1978) have criticized the half wave rectifier model and have alternatively proposed the following solar wind power input function. 0
P E= VB2 sin4 -1’
THEORY
The geomagnetic field presents an obstacle to the solar wind flow. Magnetic merging in the magnetopause region is one process which would allow the solar wind to pass through (Nishida, 1978). The magnetic field lines that reconnect in the dayside region are stretched out by the solar wind into the magnetotail. Energy from the solar wind flow is thus transferred to the stretched magnetic field. The force exerted on the field lines can be expressed as B,B&, where B, is the component normal to the boundary and B, is the tangential component. The power (P) is then the force multiplied by the solar wind velocity (V). Siscoe and Crooker (1974) derived the follo~ngexpression for the relation between the power and solar wind parameters P = ;
VB:B,L,L,
where lo = IR, (R, = earth radius) and @= arctg B,/B, B, 2 0 0 = 180” arctg By/B, B, < 0. The main difference between equations (2) and (3) lies in the solar wind dependence. The former equation has a VB solar wind dependence while the later has a IiB’ dependence. ANALYSIS AND RESULTS
The present study analyses 146 reported cases of interaction regions observed during the interval 1 January 1968-December 1973 (we did not choose earlier dates because of incomplete data). Figure 1 shows how solar wind velocity, density and
(2)
18.
12
fcm-s),
CY)
500, (km/s),
(3)
z”
0 days FIG.
1. A SUPERPOSED
INTENSITY
(----;t
EPOCH
ABOUT
146
ANALYSIS
OF SOLAR WIND
TIMES OF 1~RACTtON
1968-DECEMB~
The error bar represents
VELOClTY
(‘..),
DENSITY
REGION OCCURRENCES
(---)
DURING
1973.
two times the standard
error.
AND MAGNETIC THE INTERVAL
FIELD
JANUARY
Influence
120.
of interplanetary
0
55
55.
Y 5,
5
FIG.2a. THEVARIATIONOFTHEOBSEKVEU AE INDEX(--) AND THEENERGYCOUPLINGFUNCTIONGIVENBY PERREAULTANU AKASOFU (1978) .‘.) ABOUT TIMESOF INTERACTION REGION OCCURRENCESDURING THE INTERVAL JANUARY 1968DECEMBER1973. The error bar represents two times the typical standard error. FIG 2b. VARIATIONOFTHEOBSERVED a,-INDEX (--) AND THE COMPUTED a,-INDEX ( ‘. ‘), GIVENBY SVALGAARD(1977), ABOUT TIMES OF INTERACTION REGION OCCURRENCES DURING THE INTERVALJANUARY 196%DECEMBER 1973.
interplanetary magnetic field intensity vary at times of interaction regions. The passage dates of interaction regions are used as keydates in a superposed epoch analysis. The interplanetary medium data are taken from a tape compiled by King (1975). The typical characteristics of an interaction region are clearly illustrated-the enhanced solar wind magnetic field intensity and density due to compression and an enhanced solar wind velocity when the density decreases. The responses of different geomagnetic indices to the conditions in the interaction regions are illustrated in Fig. 2 for different proposed solar wind energy coupling functions. Figure 2a compares the variation of the AEindex with the energy coupling function P, [equation (3)] using the passage dates of interaction regions as key dates in a superposed epoch analysis. The energy coupling function was calculated, for the interval 1 January 196gDecember 1973, using 3-h solar wind data from King (1975). The 1-h A&index was averaged to give a 3-h index. The superposed epoch analysis (Fig. 2a) shows how the function P, and AE-index abruptly increase at times of interaction occurrences. It should
interaction
regions
1543
be observed that the energy coupling function P, depends on B2 and not as often stated on B. This explains why the P, function goes to high values especially at the interaction regions. In support of the above, we note that Clauer et al. (198 1) found that the P, power parameter (which depends on 1/B* in the solar wind) shows a weaker relationship to aurora1 zone geomagnetic activity than a power parameter having a VB, solar wind dependence. Figure 2b is a superposed epoch analysis in which we compare the variation of the a,,,-index with a computed a,-index, based on Svalgaard’s equation (l), at times of interaction region occurrences during the interval 1 January 1968-1973. Svalgaard’s index whichdepends on the dynamic pressure nVz, influx of merging interplanetary magnetic field lines BV, the angle between the interplanetary magnetic field and the geomagnetic field, the variability of the field and the geometry of the dipolar magnetic field, shows a very good fit with the data. The computed index closely follows the observed a,,,-index both for high and low values of the geomagnetic index. In a further study we investigated if there is an influence of the interaction regions on the tropospheric circulation. Markson (1978) has suggested that the atmospheric electric global circuit (Fig. 3b) is the missing physical link in sun-weather relations. Markson proposes that cosmic rays ionize the air above thunderstorm areas, and thereby change the atmospheric electric field. Markson also suggests that an intensified atmospheric electric field may cause more thunderstorm activity. It is therefore of interest to look for a possible influence of the interaction regions on the cosmic rays. Figure 3a shows that the cosmic ray flux intensity at Earth increases a few days before the interaction region arrives. However, at times of interaction region occurrences, the cosmic ray flux shows a drastic decrease. This decrease in cosmic ray flux would lead to a decrease of ionization and a decrease of the atmospheric electric field. According to Markson (1978) such a decrease in the atmospheric electric field could lead to a subsequent decrease in the worldwide thunderstorm intensity. In view of the above we have investigated the vorticity area index (VAI) at times of interaction region occurrences during the winter months of 1968-1973. The winter months were chosen because the VA1 in winter shows the largest response to a solar signal. Wilcox et al. (1975) mention that in winter the temperature difference between the Earth’s polar regions and equatorial regions are at maximum. This intensifies the circulation of the atmosphere and produces the largest stresses on it. The atmosphere is thus in winter possibly more susceptible to certain types
1544
H. LUNDSTEDT
interaction regions. Since the keydates of interaction region passages have been selected independently of the keydates ofsector boundary passages, our result can be considered as an independent confirmation of the VAIeffect found by Wilcox et al. (1973). After 1973 the processing of the 500-mb height grids, prepared by the National Meteorological Center, has changed. According to Wilcox et al. (1982) the increased smoothing has made it impossible to further study the relation between the crossing of sector boundaries and VAI. We therefore expect that this also should be true for our result. Recently Bhatnagar et al. (1982), using arrival dates of interaction regions as keytimes, performed a superposed epoch analysis of the zonal and meridional kinetical energy and the square of vorticity of the main motion at 500 mb height. They found no relationship between the interaction regions and the atmospheric parameters. It is therefore interesting that we found a relationship, even if not very strong, between the VA1 and the passage of interaction regions. The relationship is in accordance with the mechanism proposed by Markson (1978).
a
SUMMARY
FIG. 3a. A SUPERPOSED EPOCH ANALYSIS OF SOLAR WIND VELOClTY(-)ANDTHEDAILYMEANSOFNEUTRONMONITOR COUNTS
(----)
ABOUT
OCCURRENCES
DURING
77
TIMES OF
THE
MONTHS
INTERACTION
REGION
NOVEMBER-APRIL
OF
1968-1973. The error bar represents
two times
the typical
standard
error.
FIG. 3b. THE ATMOSPHERIC ELECTRICAL GLOBAL CIRCUIT. Thunderstorms atmospheric
represent ionization
the
is caused
electric
generator.
The
cosmic rays (after
by galactic
R. Markson, 1978). FIG.
3c.
A
VELOCITY (--)
SUPERPOSED
EPOCH
ANALYSIS
AND THJ?VORTICITY AREA
OF SOLAR
(VAI) ABOUT
WIND TIMES
OF 17 INTERACTION REGION OCCURRENCES DURING THE MONTHS NOVEMBER-APRIL The error
bar represents
OF 1968-1973.
two times
the typical
standard
error.
of solar influences.7 The superposed epoch analysis, Fig. 3c, shows the variation of VA1 at times of interaction regions. The well-known 10% dip in the VA1 which according to Wilcox et al. (1973) occurs about 1 day after a sector boundary passage, in our study coincides very nearly with the arrival times of
t Some summer.
sun-weather
studies
find other
responses
greater
in
The response of different solar wind-magnetosphere energy coupling function to variations in the interaction regions are investigated. The empirical coupling function proposed by Svalgaard fits the empirical data well and is easily physically understood. The present study indicates that the so*called P,function does not fit the observed data very well. We suggest that the well-known dip in the VA1 about 1 day after a sector boundary crossing is caused by an interaction region modulated decrease in cosmic ray flux. Acknowledgement-This
study was partly carried out during a for Plasma Research, Stanford, U.S.A. The author remembers with great joy the encouragement and guidance that the late J. M. Wilcox so kindlyofferedmeduringmyvisit.1 thankP.H. ScherrerandC. R. Clauer for many interesting and stimulating discussions about magnetospheric physics and sun-weather relations. I also thank J. 0. Stenflo and B. A. Lindblad for valuable comments on the manuscripts. 2-year
visit
to the Institute
REFERENCES Bhatnagar, V. P., Jacobsson, T. and Rosenberg, R. L. (1982) Planet. Space Sci. 30, 1079. Burlaga, L. F. (1974) J. geophys. Res. 79, 3717. Clauer, R. C., McPherron, R. L., Craig, S. and Kivelson, M. G. (1981) Geophys. Res. Lett. 8, 915. Dryer, M., Smith, Z. K., Smith, E. J., Mihalov, J. D., Wolfe, J. H., Steinolfson, R. S. and Wu, S. T. (1978) J. yeophys. Res. 83, 4347.
Influence
of interplanetary
Gosling, J. T., Asbridge, J. R., Bame, S. J. and Feldman, W. C. (1978) J. geophys. Res. 83, 1401. King, J. H. (1975) Interplanetary Magnetic Field Dam Book, NASA-NSSDC 75-04. National Space Science Data Center, Greenbelt, Maryland. Markson, R. (1978) Nature 273, 103. Markson, R. (1979) Atmospheric electricity and the sunweather problem, in Solar-Terrestrial influence on Weather and Climate (Edited by McCormac, B. M. and Seliga, T. A.). D. Reidel, Dordrecht: Nishida, A. (1978) Geomagnetic Diagnosis of the Magnetosphere (Edited by Roederer, J. Gland Wasson, J. T.). Springer, New York. Olson, R. H., Roberts, W. 0. and Zerefos, C. S. (1975) Nature 257, 113. Perreault, P. and Akasofu. S.-I. (1978) JI R. astr. Sot. 54, 547.
interaction
regions
1545
Rosenberg, R. L. and Coleman, P. J., Jr. (1980) J. geophys. Res. 85, 3021. Siscoe, G. L. (1972) J. geophys. Res. 77,27. Siscoe, G. L. and Crooker, N. (1974) Geophys. Res. Lett. 1,17. Svalgaard, L. (1977) Geomagnetic Actiuity: Dependence on Solar Wind Parameters, A Monograph from Skylab Solar Workshop 1, Coronal Holes and fiigh Speed Wind Streams (Edited by Zirker, J. B.). Colorado Associated University Press, Boulder, Colorado. Wilcox, J. M., Scherrer, P. H. and Hoeksema, J. T. (1982) SUIPR Reoort No. 945. Wilcox, J. M.1 Scherrer, P. H., Svalgaard, L., Roberts, W. 0. and Olson, R. H. (1973) Science 180, 185. Wilcox, J. M., Svalgaard, L. and Scherrer, P. H. (1975) Nature 255, 539.
1984
0032LO633~84$3.00 + O.CfJ Per&mm Press Ltd.
INFLUENCE OF INTERPLANETARY INTERACTIO-N REGIONS ON GEOMAGNETIC DISTURBANCES AND TROPOSPHERIC CIRCULATION HENRIK LUNDSTEDT
Lund Observatory,
Box 1107, S-221 04 Lund, Sweden
(Received injnalform
30 April 1984)
Abstract--Proposed solar wind-ma~ctosphere energy coupling functions are studied. An empirical formula proposed by Svaigaard (1977) is found to predict the geomagnetic activity quite well. The influence of solar wind interaction regions on the tropospheric circulation, through a suggested cosmic ray mechanism, was investigated. The cosmic ray intensity at Earth clearly showed a decrease at the time of passage of an interaction region. It is suggested that the well-known dip in the Vorticity Area Index may be caused by an interaction-modulated decrease in cosmic ray intensity.
INTRODUCTION
parameters
It is well known that when a fast solar wind stream overtakes the trailing edge of a preceding, slow stream, a characteristic interaction region develops. The expression “interaction region” implies that within such a region fast and slow plasmas are interacting, by means of a transfer ofmomentum and energy, involving large amplitude waves and shocks. An interaction region is characterized by enhanced field magnitude (S), particle density (n) and temperature (7’) (Si&oe, 1972). These characteristics can be used to identify the interacting zones. Rosenberg and Coleman (1980) found 234 interaction regions, during the interval 1964 1973, using the enhanced interplanetary magnetic fieid, as a signal of an interaction region. Within the interaction region, there is a narrow zone called the interface, where the density drops sharply, proton tem~rature increases sharply, magnetic fields maximize and P = nkT+B2/8n peaks (Siscoe, 1972; Burlaga, 1974 ; Gosling et al., 1978). The gradient on the two sides of the interface results in stresses that are exerted both inward and outward. Theoretical studies of interaction regions have also been performed (Dryer et al., 1978). The physical processes involved in interaction region are now quite well understood. The Earth’s magnetosphere can be considered as a filter with the solar wind as the input and with the geomagnetic activity as output. Several different geomagnetic activity indices are presently used. The relationships between these indices and the power input are not clear. The index a, appears to increase linearly with power input. Svalgaard (1977) found the following relationship by studying the different solar wind
separately
1.157 x* a, = 6.6q(J; c() - 32 * K 21 105 (1 -i-3 cos.2 $)2’”
(l)
where nvg is the dynamic pressure, BI$ the influx of interplanetery magnetic field lines, TVis the angle between the interplanetary field and the geomagnetic field of the dayside magnetopause, f describes the variability of 8, the function q off and a is a fourth order polynom and $ is the angle between the solar wind flow direction and the dipole axis. It has been proposed that cosmic rays, modulated by variations ofsolar wind velocity and magnetic field may influence the tropospheric circulation. As a measure of the tropospheric circulation Roberts and Olson introduced the “vorticity area index” (VAI), a number that is high, when the area covered by relatively intense cyclonic circulation near 500 mb is large. Storminess is statistically correlated with high atmospheric vorticity. Wilcox et al. (1973) reported a sun--weather effect. They showed that the VAIt in the Northern Hemisphere during the winter period exhibits a pronounced dip a day or two after the boundary of an interplanetary sector sweeps past the Earth. Markson (1978) has proposed a mechanism, which involves the atmospheric electrical global circuit, to explain the observed sun-weather effect. In this theory current flows upward to the ionosphere from thunderstorms and then back through the fair-weather part of the atmosphere.
7 The VA1 should not be equated with “storminess” per se nor do Wilcox et al. claim this. Higher positive vorticity is however associated with disturbed weather conditions.
1541
H.
1542
LUNDSTEDT
Ionization above the thunderstorm area, and thus the magnitude of the atmospheric electric field, is changed due to variations in the cosmic ray intensity. Markson (1978, 1979) suggests that an intensified atmospheric electric field could cause more storminess and if so an enhanced VA1 would be expected. The cosmic ray intensity is increased after a solar flare and decreased after an interaction region passage. We thereforeexpect an increase of VA1 after a solar flare and a decrease of VA1 at times of interaction regions. Olson et al. (1975) reported a peak of VA1 on the first and second day of a flare. Markson’s work offers an explanation for how solar variability controls the electrification of the Earth’s atmosphere. It also proposes ways by which the changing Sun in turn may affect the weather.
where L, = L, = B: = V=
length of the magnetotail length of merging line merging component of magnetic field solar wind velocity.
In the so-called half wave rectifier model ST has been replaced by B, =O, B,=-BZ,
B, > 0 Bz
where B, is the North-South component of the interplanetary magnetic field. Perreault and Akasofu (1978) have criticized the half wave rectifier model and have alternatively proposed the following solar wind power input function. 0
P E= VB2 sin4 -1’
THEORY
The geomagnetic field presents an obstacle to the solar wind flow. Magnetic merging in the magnetopause region is one process which would allow the solar wind to pass through (Nishida, 1978). The magnetic field lines that reconnect in the dayside region are stretched out by the solar wind into the magnetotail. Energy from the solar wind flow is thus transferred to the stretched magnetic field. The force exerted on the field lines can be expressed as B,B&, where B, is the component normal to the boundary and B, is the tangential component. The power (P) is then the force multiplied by the solar wind velocity (V). Siscoe and Crooker (1974) derived the follo~ngexpression for the relation between the power and solar wind parameters P = ;
VB:B,L,L,
where lo = IR, (R, = earth radius) and @= arctg B,/B, B, 2 0 0 = 180” arctg By/B, B, < 0. The main difference between equations (2) and (3) lies in the solar wind dependence. The former equation has a VB solar wind dependence while the later has a IiB’ dependence. ANALYSIS AND RESULTS
The present study analyses 146 reported cases of interaction regions observed during the interval 1 January 1968-December 1973 (we did not choose earlier dates because of incomplete data). Figure 1 shows how solar wind velocity, density and
(2)
18.
12
fcm-s),
CY)
500, (km/s),
(3)
z”
0 days FIG.
1. A SUPERPOSED
INTENSITY
(----;t
EPOCH
ABOUT
146
ANALYSIS
OF SOLAR WIND
TIMES OF 1~RACTtON
1968-DECEMB~
The error bar represents
VELOClTY
(‘..),
DENSITY
REGION OCCURRENCES
(---)
DURING
1973.
two times the standard
error.
AND MAGNETIC THE INTERVAL
FIELD
JANUARY
Influence
120.
of interplanetary
0
55
55.
Y 5,
5
FIG.2a. THEVARIATIONOFTHEOBSEKVEU AE INDEX(--) AND THEENERGYCOUPLINGFUNCTIONGIVENBY PERREAULTANU AKASOFU (1978) .‘.) ABOUT TIMESOF INTERACTION REGION OCCURRENCESDURING THE INTERVAL JANUARY 1968DECEMBER1973. The error bar represents two times the typical standard error. FIG 2b. VARIATIONOFTHEOBSERVED a,-INDEX (--) AND THE COMPUTED a,-INDEX ( ‘. ‘), GIVENBY SVALGAARD(1977), ABOUT TIMES OF INTERACTION REGION OCCURRENCES DURING THE INTERVALJANUARY 196%DECEMBER 1973.
interplanetary magnetic field intensity vary at times of interaction regions. The passage dates of interaction regions are used as keydates in a superposed epoch analysis. The interplanetary medium data are taken from a tape compiled by King (1975). The typical characteristics of an interaction region are clearly illustrated-the enhanced solar wind magnetic field intensity and density due to compression and an enhanced solar wind velocity when the density decreases. The responses of different geomagnetic indices to the conditions in the interaction regions are illustrated in Fig. 2 for different proposed solar wind energy coupling functions. Figure 2a compares the variation of the AEindex with the energy coupling function P, [equation (3)] using the passage dates of interaction regions as key dates in a superposed epoch analysis. The energy coupling function was calculated, for the interval 1 January 196gDecember 1973, using 3-h solar wind data from King (1975). The 1-h A&index was averaged to give a 3-h index. The superposed epoch analysis (Fig. 2a) shows how the function P, and AE-index abruptly increase at times of interaction occurrences. It should
interaction
regions
1543
be observed that the energy coupling function P, depends on B2 and not as often stated on B. This explains why the P, function goes to high values especially at the interaction regions. In support of the above, we note that Clauer et al. (198 1) found that the P, power parameter (which depends on 1/B* in the solar wind) shows a weaker relationship to aurora1 zone geomagnetic activity than a power parameter having a VB, solar wind dependence. Figure 2b is a superposed epoch analysis in which we compare the variation of the a,,,-index with a computed a,-index, based on Svalgaard’s equation (l), at times of interaction region occurrences during the interval 1 January 1968-1973. Svalgaard’s index whichdepends on the dynamic pressure nVz, influx of merging interplanetary magnetic field lines BV, the angle between the interplanetary magnetic field and the geomagnetic field, the variability of the field and the geometry of the dipolar magnetic field, shows a very good fit with the data. The computed index closely follows the observed a,,,-index both for high and low values of the geomagnetic index. In a further study we investigated if there is an influence of the interaction regions on the tropospheric circulation. Markson (1978) has suggested that the atmospheric electric global circuit (Fig. 3b) is the missing physical link in sun-weather relations. Markson proposes that cosmic rays ionize the air above thunderstorm areas, and thereby change the atmospheric electric field. Markson also suggests that an intensified atmospheric electric field may cause more thunderstorm activity. It is therefore of interest to look for a possible influence of the interaction regions on the cosmic rays. Figure 3a shows that the cosmic ray flux intensity at Earth increases a few days before the interaction region arrives. However, at times of interaction region occurrences, the cosmic ray flux shows a drastic decrease. This decrease in cosmic ray flux would lead to a decrease of ionization and a decrease of the atmospheric electric field. According to Markson (1978) such a decrease in the atmospheric electric field could lead to a subsequent decrease in the worldwide thunderstorm intensity. In view of the above we have investigated the vorticity area index (VAI) at times of interaction region occurrences during the winter months of 1968-1973. The winter months were chosen because the VA1 in winter shows the largest response to a solar signal. Wilcox et al. (1975) mention that in winter the temperature difference between the Earth’s polar regions and equatorial regions are at maximum. This intensifies the circulation of the atmosphere and produces the largest stresses on it. The atmosphere is thus in winter possibly more susceptible to certain types
1544
H. LUNDSTEDT
interaction regions. Since the keydates of interaction region passages have been selected independently of the keydates ofsector boundary passages, our result can be considered as an independent confirmation of the VAIeffect found by Wilcox et al. (1973). After 1973 the processing of the 500-mb height grids, prepared by the National Meteorological Center, has changed. According to Wilcox et al. (1982) the increased smoothing has made it impossible to further study the relation between the crossing of sector boundaries and VAI. We therefore expect that this also should be true for our result. Recently Bhatnagar et al. (1982), using arrival dates of interaction regions as keytimes, performed a superposed epoch analysis of the zonal and meridional kinetical energy and the square of vorticity of the main motion at 500 mb height. They found no relationship between the interaction regions and the atmospheric parameters. It is therefore interesting that we found a relationship, even if not very strong, between the VA1 and the passage of interaction regions. The relationship is in accordance with the mechanism proposed by Markson (1978).
a
SUMMARY
FIG. 3a. A SUPERPOSED EPOCH ANALYSIS OF SOLAR WIND VELOClTY(-)ANDTHEDAILYMEANSOFNEUTRONMONITOR COUNTS
(----)
ABOUT
OCCURRENCES
DURING
77
TIMES OF
THE
MONTHS
INTERACTION
REGION
NOVEMBER-APRIL
OF
1968-1973. The error bar represents
two times
the typical
standard
error.
FIG. 3b. THE ATMOSPHERIC ELECTRICAL GLOBAL CIRCUIT. Thunderstorms atmospheric
represent ionization
the
is caused
electric
generator.
The
cosmic rays (after
by galactic
R. Markson, 1978). FIG.
3c.
A
VELOCITY (--)
SUPERPOSED
EPOCH
ANALYSIS
AND THJ?VORTICITY AREA
OF SOLAR
(VAI) ABOUT
WIND TIMES
OF 17 INTERACTION REGION OCCURRENCES DURING THE MONTHS NOVEMBER-APRIL The error
bar represents
OF 1968-1973.
two times
the typical
standard
error.
of solar influences.7 The superposed epoch analysis, Fig. 3c, shows the variation of VA1 at times of interaction regions. The well-known 10% dip in the VA1 which according to Wilcox et al. (1973) occurs about 1 day after a sector boundary passage, in our study coincides very nearly with the arrival times of
t Some summer.
sun-weather
studies
find other
responses
greater
in
The response of different solar wind-magnetosphere energy coupling function to variations in the interaction regions are investigated. The empirical coupling function proposed by Svalgaard fits the empirical data well and is easily physically understood. The present study indicates that the so*called P,function does not fit the observed data very well. We suggest that the well-known dip in the VA1 about 1 day after a sector boundary crossing is caused by an interaction region modulated decrease in cosmic ray flux. Acknowledgement-This
study was partly carried out during a for Plasma Research, Stanford, U.S.A. The author remembers with great joy the encouragement and guidance that the late J. M. Wilcox so kindlyofferedmeduringmyvisit.1 thankP.H. ScherrerandC. R. Clauer for many interesting and stimulating discussions about magnetospheric physics and sun-weather relations. I also thank J. 0. Stenflo and B. A. Lindblad for valuable comments on the manuscripts. 2-year
visit
to the Institute
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