Statistical analysis of magnetically closed structures

Planet. Printed

Space Sci., Vol. 35, No. 6, pp. 727-736, in Great Britain.

STATISTICAL

00324633/87%3.00+0.00 Pergamon Journals Ltd.

1987

ANALYSIS OF MAGNETICALLY STRUCTURES

CLOSED

A. GERANIOS

Nuclear

and Particle Physics Section, Department of Physics, University Panepistimioupoli Kouponia, 157 71 Athens, Greece

of Athens,

(Received in final form 24 November 1986) Abstract-An attempt is made for a better identification of magnetically closed structures in the solar wind, using the low proton temperatures between 0.3 and 1.0 astronomical unit (a.u.). The analysis is based on H&OS-I hourly data, which cover the period from its launch (December 1974) up to the end of 1980. All hourly data are examined in order to detect low temperature regions embeded within closed magnetic field lines. The solar wind proton parameters, as the temperatures, densities, velocities, components of the Interplanetary Magnetic Field (IMF) and ratios of magnetic to plasma pressures, were checked, being indicative of such configurations. The temperature variation (7’) with radial distance (r) of the “hot” component in the solar plasma (u > 550 km s- ‘) does not show an adiabatic variation (T - r-J!3 ), but a variation with an exponent close to 0.75 (T - rm 3,4). On the contrary, the “cold” component (c < 550 km s- ‘) shows a variation close to the adiabatic. In almost all events found, the two main criteria for closed structures (which are reduced temperatures and closed magnetic fields) are fulfilled

structure. This cool mass expands moving away from the sun and reaches a size of about 0.25 a.u. at the distance of 1 a.u. (Burlaga et al., 1981 ; Bame et al., 198 1; Iucci et al., 1982 ; Klein and Burlaga, 1982). Measurements at 2 a.u. (Vqqev) showed an increase of its size by a factor of 2, expanding further at greater distances (for example, at 4 a.u., see Burlaga and Behannon, 1982). Most of the authors suggested a magnetically closed structure to support their observations (also calling it “blob”). On the other hand, it has been shown that in several cases the observations could even be supported by an open, rather than closed, magnetic field model (Geranios, 1981b). This assumption is based on the low-temperature plasma observations, which mainly appeared during the decreasing phase of a solar wind stream. The basic idea is that, because in this phase the solar wind is successively decelerating, the volume of the plasma in front of the stream is generally increasing. This “expansion” causes a cooling of the plasma which at the same time is heated by the solar corona through the electrons coming via the open magnetic field lines. These lines connect the plasma with the hot corona. The balance of these two effects causes a final cooling of this plasma. It seems that the majority of low-temperature observations is of this feature (Geranios, 1978, 1980, 1981a,b, 1982).

1. INTRODUCTION

Since 1973, when Gosling et al. (1973) identified the Very Low Temperatures (VLTs) in the solar wind, this phenomenon has been approached in different ways. Montgomery et al. (1974), using 3-hourly electron data from IMP and P’ELA geocentric satellites, defined an upper threshold of 0.5 x lo5 K for the electron component of the solar wind at 1.0 a.u. as the limit for low temperature electrons. Other authors defined a corresponding limit of 1.5 x lo4 K for the proton component at the same distance in order to identify the same low-temperature regions, measuring protons instead of electrons (Feldman et al., 1973; Gosling and Roelof, 1974; Krimigis et al., 1976). With the launch of the heliocentric satellite Helios-1, this phenomenon could be observed even at closer distances. Due to the positive radial temperature gradient toward the sun, new normalized low-temperature thresholds should be calculated as a function of distance. The models, which could explain the locally reduced plasma temperatures in the interplanetary space, are based on the assumption that these regions form a magnetically closed structure, which is disconnected from the sun’s magnetic field, causing a lack of any heat transport from the hot solar corona to this closed 721

728

A.

GERANIOS

Two other cases of cool plasma also appear : (a) after a shock front and (b) in the neighbourhood of IMF boundaries. Case (a), the most representative among the others, is the appearance of a closed structure, some hours after the passage of a shock front, being analysed, among others, by Sanderson et a/. (1983). Case (b) is associated with a stream interface or with a cold magnetic enhancement (Schwenn et al., 1978; Burlaga et al., 1981). As shown by Burlaga (1983) and Wilson and Hildner (1984) cases (a) and (b) are not clearly defined, and this distinction is not unique. 2. DATA ACQUISITION

As a source of Helios hourly data, the standard magnetic tapes, red by a UNIVAC in the Data Handling Division in Goettingen Max-Planck-Institut, were used. These tapes include on the whole 80 solar wind plasma and magnetic field parameters. Among these parameters, the 20 most important were retained for our analysis and were transfered as one file to the mass storage disc of an INTERDATA machine in the Lindau Max-Planck-Institut. The whole, including 5-y data, was chopped into 60, in order to fit to 60 ordinary 5lj” floppy disks (one month of data each). The files were further transferred to an LSI-11 machine of the same institute. Finally, by synchronizing the LSI-11 with an Apple II+, we were able to copy all 60 files to 60 floppy disks. The hardware connection of the two machines was via a serial interface (RS232). All the numeric data were stored as strings. Use and analysis of the data was a matter of simple BASIC and machine language programs. The scope of this procedure is the possibility of analysing a large amount of data by small and cheap personal computers. During data transfer from the LSI-11 to the Apple computer, small problems arose due to the different speeds of the computers. The problems were solved by retarding the output of the faster computer (LSI) by a FORTRAN program, before being accepted by the slower (Apple). The most reliable data transfer speed was found to be 300 baud. 3. OBSERVATIONS

The found shows about The

Time (hours) FE. ~.TIME DISTRIBUTION OF

VLTs

1.5 x IO“ K at 1 a.u. has been selected as the mean temperature of 30 VLTs analysed in the past (Geranios, 1978) and which is in accordance with that suggested by other authors also, referenced in the Introduction. This limit is about seven times less than the mean temperature of the normal ambient solar wind (Geranios, 1982). As VLTs are not only observed at 1 a.u. but even at closer distances to the sun (0.3 a.u.), the proton temperature radial gradient has been used for normalization. This gradient (k) is involved in the relation T/T, = (r/r,) -‘,

(1)

To being the temperature at the distance of rO= 1 a.u. The availability of the data for several distances is shown in Fig. 2. The histogram indicates a maximum of data, corresponding to the longest distance (0.951.OOa.u.).

AND DATA ANALYSIS

duration of the closed structures has been to vary from several hours to 4 d. Figure 1 this duration, indicating the most probable of 15 h. threshold of low proton temperatures of

Distance (a u/100) FIG. 2. DISTRIBUTION OF THE COLU SOLAR WIND MENTS ACCORDING

TO KADIAL

DISTANCES

MEASURE-

FROM THE SUN.

729

Magnetically closed structures Because temperature gradients depend on solar wind speed, for each hourly tem~rature value the corresponding speed is considered in order to apply the appropriate temperature gradient. The values of the gradients, corresponding to a certain speed interval, are shown in Table 1 and are recalculated with a least-squares fit. In the calculations, periods characterized by highly perturbed solar wind (as shocks, for example), have been removed. The whole speed interval (250-800 km s- ‘) is divided into 11 parts. For the slow solar wind (v < 550 km s- ‘) the gradient is generally near to - 514, a value which is close to the adiabatic expansion. This value contradicts that suggested for protons by Sturrock and t-fartle (1966). They calculated the value from the cold solar wind in radial distances, using the conservation equations of mass, momentum and energy for a two-fluid solar wind model (electrons f protons). For the fast solar wind (v > 550 km s-l), the gradient is close to -3/4. The values shown in Table 1 are slightly different from those calculated by Schwenn et al. (1978). This discrepancy may be due to the different total analysed period and to the removal of some highly perturbed periods. Due to the large errors, the classification could be simpler. Apart from the temperature threshold criterion, to identify a VLT region in the solar wind other parameters have also been taken into account. The most important among them is the intensity of the magnetic field (B) with its three components (B,, By, BZ; Fig. 3). The temporal evolution of the magnetic field could indicate whether the cold region is surrounded by closed magnetic field lines or not. The other indicative parameters, speed and density, are considered without the definition of any limits. Finally, for closed structure candidates, the ratio criterion of the magnetic to plasma pressure

TABLE

1. RADIAL PROTON PERATUREGRADIENTS

V(kms-‘) 250-300 301-350 351400 401450 45 l-500 501-550 551-500 601-650 651-700 701-750 751-800

TEM-

Gradient 1.34+0.09 1.20+0.12 1.22+0.10 1.05+0.06 1.00+0.03 1.00-+0.03 0.73io.05 0.70+0.04 0.75 + 0.06 0.67 f0.04 0.68 i 0.03

Ftcr.3.

THE~OMPO~ENTSOFT~I.~ERPLA~TARYMAG~~IC FIELD.

(B’: 8n/nkT) has been strongly taken into account (n being the number density of protons and k, the Boltzmann constant). Due to the expansion of the closed region, as it is convected outward by the solar wind, and assuming conservation of mass inside the structure, a greater magnetic to plasma pressure should dominate this region. In fact, the majority of the cases show magnetic pressures much larger than plasma pressures (exceeding in some cases a ratio ten to one). 4. STATISTICAL BEHAVIOUR MAGNETIC

OF

FIELD, DENSITY AND

PRESSURE RATIO

After normalizing all 5-y temperature data, according to Table 1 and equation (l), a number of about 600 h appeared as very low temperatures. In order to get the distribution of the above three parameters as a function of their VLTs, the corresponding scatter plots are presented. A short description of the scatter plots follows*

(11 The amplitude of the magnetic field shows that the majority of the VLTs were characterized by fields with amplitudes of about 1Oy(Fig. 4). (2) Considering the same VLTs, the corresponding proton density varies from very low (3 cm-‘) to very high values (50 cm- 3; Fig. 5). (3) The speed fluctuates in a low range from 230 to 500 km s- ’ (Fig. 6). A similar range has also been found by Freeman and Lopez (1985) after analysis of proton temperatures below 1.5 x lo4 K. (4) Generally, the magnetic to plasma pressure ratios are widely spread, but they show larger magnetic to plasma pressures, indicating an expansion of the cold regions (Fig. 7).

730

A.

GERANIOS

Beta vs temperature

Speed vs temperature

Temperature (x loo0 K) Tempemture (x 1ooO K) FIG. 4. MAGNETIC

FIG.~.SOLAR

WIND SPEED FOR LOW TEMPERATURES.

FIELD AMPLITUUE vs LOW TEMPERATURES.

As for the theoretical predictions for a magnetically closed structure, one could expect :

(1) high magnetic field amplitude (B), (2) low proton density (because of the expansion,

if

the mass inside the structure is conserved), (3) low proton speed, (4) high pressure ratio (B2 : 8rr/nk T) and (5) characteristic time evolution of the IMF direction. Generally, the experimental observations and the theoretical predictions are in agreement, except for

the density behaviour. The magnetic field shows a mean value of 10~. This could be considered as an elevated value, since normally it is about half this (5~). We could not say the same for the density, which on average has a moderate value (10 cm- ‘). This contradicts the expected low density required by the closed model. On the other hand, the proton speed is in agreement with the temperature variation, mainly due to the conservation of mass and energy. Finally, the pressure ratio is in fact much larger than unity, and this is in accordance with the theoretical suggestions.

Ratio vs temwrature

Temperature (x 1000 K) Temperature (x 1000 K) FIG. 5. PROTON DENSITYFOR LOWTEMPERATURES.

FIG.

7.

MAGNETIC

TO PLASMA PRESSURES TEMPERATURES.

FOR

LOW

Magnetically closed structures A time-profile of the solar wind parameters in normal conditions and for one complete trajectory of H&x-f is shown in Fig. 8. Very &se to the sun, the amplitude of the magnetic field (B) shows a maximum, due to the fieId gradient. The inclination, O,,, of the magnetic field mostly fluctuates between 0” and 45”, remaining for the whole &month period above the ecliptic plane. The azimuth of the field, dR, clearly indicates the passage of successive IMF boundaries. This is shown by the alternative AWAY and TOWARD directions. As for the plasma parameters, the speed profife depicts many solar wind streams with maximum values around 700 km sP I_The proton density shows a maximum close to the sun, like the magnetic field.

731

This maximum reaches values of 200 protons CM-‘. The fluctuation of the density is generafly anticorrelated with the speed. This is due to the conservation of mass of the general solar wind. The temperature more or less follows the speed variation. 5. EXAMPLES

OF MOST INDICATIVE

LOWvTEMPERATURE 5.1.

REGIONS

ZMF

From the low-temperature cases, which show a deepening and a successive rising of the proton temperature, reaching the minimum value required by equation (I), we present the most characteristic case. This presentation depicts the time profile of magnetic

A. GERAN~OS

732

temperature. We consider as the most dominant parameter for a closed structure 70 Ithe appropriate time-variation of the magnetic field direction. Before going on with the description of the VLTs, we should mention the field behaviour required by the model. While Helios is transversing such a struciure, one should observe a rotation of the azimuth by 180” if the satellite crosses it centrally and less if laterally. Simuftaneously, the inclination of the field should vary within opposite directions. This feature is an indicative criterion for magnetj~aliy cfosed structures. For such cases, if one plots the profile of the magnetic field components, one should expect a rotation parallel to the .r-y, X-Z, y-z plane or any other inclined plane. A very simplified model is shown in Fig. 9. The dashed line represents two possible trajectories of h -6OHelz’os. One is central and the other intersects the structure laterally. In the first case, the azimuthal vari-120 ation of the magnetic field is expected to be 180”. -Iaoc_ In the second, it should be less. The first VLT example (75.63.11-75.64.6) shows a rather intense field (6@) and a characteristic variation of its direction. Very clear is the change of the direction from below to above the ecliptic plane by 1OO” (Fig. HI). The second event (7~.6~.22-7~.62.3) is the most representative among all. Its duration is rather long (30 h) and the field shows initially a sharp increase and a further progressive decrease. Both azimuth (PHI) and inclination (EPS) vary within a very wide range. At the end of day 61, Helios had probably -6O/passed through an IMF sector boundary, as shown by the PHI plot. (Fig. 11). FIG. 1&ANEXAMPLEQPA The last example is drawn from measurements at wind (B, AMPLITUDE fieM, speed, density and

Event 1 T5.63.f I-7544.#

. /’

CI.OSEDSTRUCTUREINTHESOLAR OF IMF; PHI, AZIMUTH AND EPS, lNCtlNATlON'~0 ‘rI-IE ECLIPTIC PLANE).

0.44 a.u. (79.14S.G79.148.18).

The held shows a maximum with a rather elevated value (30~). The inclination changes from below to above the ecliptic plane, while the azimuth oscillates within 50” (Fig. 12).

Fro. 9.A

VERY SIMPLIFI~~MODEL FORCLOSED STRUCTURES.

5.2. Plasma parameters Of the same examples, the other three plasma parameters are shown in the following figures. The first event, except for the expected behaviour of the speed and temperature, shows an increase instead of a decrease in density (Fig. 13). The second event, the longest, shows simitar characteristics with the previous one. except for the slower

Magnetically closed structures

733 Event 3 79.!48.6-79.148.lS

0

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I 9

I 12

I I5

I 16

I 21

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I 27

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-x)

-4OC Time FIG.

WIND

I+

7

9

II

I3

hod

AN EXAMPLEOFA CLOSEDSTRUCTUKEINTHESOLAR (f?,AMPLITUDE OF IMF; Pm, AZIMUTH AND EPS, INCLINATION TO THE ECLIPTIC PLANE).

Time

Cf hours)

11.

solar wind. In this example, the density decreases as expected by the model (Fig. 14). The third event shows a very large decrease in density and temperature with local oscillations in speed (Fig. 15). 5.3. Closed fields Because the proposed model is based on the closed magnetic field, we present in the following graphs the variation of the field components for the events discussed so far. Figure 16 shows the first event in which the magnetic field is rotated by 2~ in a plane parallel to the

FIG.

WIND

12. AN EXAMPLEOF

A CLOSED STRUCTUREIN THESOLAR (B, AMPLITUDE OF IMF, PHI, AZIMUTH AND EPS INCL~NAT~O~TOTHEECL~PTIC PLAh'E).

ecliptic plane. Figure 17 shows the second example characterized by a complete rotation in the ecliptic plane. Figure 18 shows the last example. In this case, the rotation of the field is perpendicular rather than parallel to the ecliptic plane. It has to be noted that for all examples, the three field components were checked for a rotational feature. Finally, as an additional criterion for a VLT, the ratio of magnetic to plasma pressure and the momentum flux (nm,VZ) was also considered (77.30& 77.30.17). The period is indicated by the two vertical dashed lines, see Fig 19 (mp is the proton mass and Y

A.

GERANIOS

Event I

Event2

75.63. I l-75.64.06

70.60.22-70.62

0

3

3

6

9

12

15

16

21

24

27

3O

3

6

9

12

15

16

21

24

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30

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II 21

24

I 27

)_ 30

.

I\

90-

70-

50 30-

,q

lOI 0

4

icbP-h.#yJ--

8

12

\

16

20

.-...

24

0

130-. II0 --\-

7O-

.\.

5030-

1 -, +=&+*QXf=--*

IO0 Time if

hours)

3

I 6

I 9

I I2

I I5

16

Time (+ hours)

FIG. 13. ANEXAMPLEOF ACLOSEDSTRUCTUREIN THESOLAR WIND (SPEED,PROTON DENSITY ANDTEMPERATURE).

FIG. 14. AN EXAMPLEOF A CLOSED STRUCTURE IN THE SOLAR WIND(SPEED,PROT~N DENSITY ANI) TEMPERATURE).

the solar wind speed). As also shown by Fig. 7, the majority of the cases shows larger magnetic than plasma pressures, indicating in this way an expansion due to the magnetic pressure. The minima of momentum flux suggests the reduced speed and density characteristic of such structures.

criteria for such structures, i.e. reduced temperatures and closed fields, were found in almost all the events. Deviations from the rule, like increases in density, shown in Fig. 13, may be due to stream interactions, which supply the structures with additional solar plasma (i.e. questionable conservation of mass). We did not think that the proposed structure in Fig. 9 was the only one which could be supported by the data. An alternative is, for example, the cylindrical shape, proposed by Burlaga et al. (1981). But, the fact that both the azimuth and the inclination of the IMF should rotate inside the structure leads to a symmetrical form. Because Helios moves close to the eclip-

6. SUMMARY

A systematic search 5-y period, apart from depicted in Figs 4, 5, 6 which can be regarded

for closed structures over a the characteristic of solar wind and 7, showed numerous cases as cold regions. The two main

Magnetically Event 3 79 148.6-79.148

closed structures

252015-

t 2 E =

IO 5%_, 0

c3 -5m= -IO-I5 -

4601 0

I 4

I 2

I 6

78.60.22-78.62.03

30-

18

560 ^

735

I

I

I

I

8

IO

12

14

-Xl-25 -30 IO

,=?

ny

.cl+

Event 2 L

I

P=

‘

i

HELIOS-I

15

20

25

30

35

13,-20 Gamma FIG. 17. AN EXAMPLEOFA

ROTATIN<; MAGNETKFIELD.

tic plane, as the other heliocentric satellites do, our data are restricted in this plane only. An off-ecliptic mission could trace such configurations in a threedimensional space, giving rise to a better identification of this phenomenon. The statistical analysis of cold solar wind speed at 1 au. by Freeman and Lopez (1985) is generally in agreement with ours. The mean density of 10 cm-’ found, may be due to a contamination of the data by periods of very low solar wind speed and therefore very low tempeature. The proton density criterion was the most unstable in the whole analysis, compared with the other four (i.e. field amplitude, direction, pressure ratio and temperature). Finally, we could summarize that :

Time ~thwrsf

Frc;. 15.

ANEXAMPLEOF A CLOSEDSTRUCTURE

IN THESOLAR WIND(SPEED, E’ROT~‘ONDENSITY AND TEMPERATURE).

& -10

L-

-20 -30 -40 -50 0 IO

‘--A. 20

x)

20

Event 3 79.148.06-79 148.19

15

HEUOS-

I dtstance 0.44

au.

Distance 0.38 a.”

40

50

0~ -50

FIG. 16.

(1) the most probable duration of a VLT region is 15 h, (2) the magnetic pressure inside is on average three times larger than the plasma pressure and (3) the mean magnetic field magnitude and solar wind speed in the structure are IOy and 320 km se ‘, respectively.

60

70

I30 90

too

Gamma

ANEXAMPLEOF A KOTATING

-2o20

. 25

30

35

40

45

50

B, - 20 Gamma MAGNETICFIELD.

FIG. ~&AN EXAMPLEOFA

ROTATIN<; MAGNETIC: FIELD

A.

736

GERANIOS

IO

c” r 5 m

“m ’ 28

I

31

February 1977

-rY

FIG. 19. AN

EXAMPLE

OF MOMENTUM

PLASMA PRESSURE

RATIO,

FLUX

AND MAGNETIC

AS A FIJNCTION

TO

OF TIME.

Our model. as well as the others. should be theoretically supported and questions which arise from the spherical structure should be answered. For example, how does a closed structure remain stable, especially in its boundary where the field is ouvosite to the main field of the s&or? What does t& structure above

Burlaga, L. (1983) Heliospheric magnetic fieIds and plasmas. Rev. Geophys. Space Phys. 21,363. Burlaga, L. and Behannon, K. (1982) Magnetic clouds : Voyager observations between 2 and 4 a.u. Solar Phys. 81, 181. Burlaga, L., Sittler, E., Mariani, F. and Schwenn, R., (1981) Magnetic loop behind an interplanetary shock : Voyager, Helios and IMP-8 observations. J. aeoohvs. Res. 86.6673. Feldman, W., Asbridge, J., Bame, S.-anh Montgome;y, M. (1973) On the origin of solar wind proton thermal anisotropy. J. geophys. Res. 78,6451. Freeman, .I. and Lopez, E. (1985) The cold Solar Wind. J. geophys. Res. 90,9885. Geranios, A. (1978) A search for the origin of very low electron temperatures. Planet. Space Sci. 26, $7 1. Geranios, A. (1980) Solar wind velocity decreases. Nuot?o Cim. 3G, 382. Geranios, A. (1981a) Plasma temperature depressions and the open magnetic field-lines model. Astrophys. Space Sci. 77, 167. Geranios, A. (1981b) Non-adiabatic expansion of low-temperature solar wind. Radial temperature gradients. J. Geophys. 49, 192. Geranios, A. (1982) Magnetically closed regions in the solar wind. Astrophys. Space Sci. 81, 103. Gosling, J., Pizza, V. and Bame, S. (1973) Anomalously low proton temperatures in the solar wind following interplanetary shock waves, evidence for magnetic bottles? J. geophys. Res. 78,2001.

Gosling, J. and Roelof, E. (1974) A comment on the detection of closed magnetic structures in the solar wind. Solar Phw. 39, 405.

a

Iucci; N., Parisi, M., Storini, M. and Villoresi, G. (1982) Inlerplanetary perturbations associated with type-IV solar Aares. Fifth Int. Symp. STP, Ottawa, Canada. Klein, L. and Burlaga, L. (1982) Interplanetary magnetic clouds at 1 a.u. J. aeoohvs. Res. 87. 613. Krimieis, S.. Sarris, “E. &l Armstrong, T. (1976) Evidence

These questions have to be answered in order to conclude what the form of the cold and magnetically

for closed magnetic loop structures-in the interplanetary medium. Trans. AGU 57, 304.

and

below

cylindrical, this cylinder

closed

the

ecliptic

rather wrap

structures

plane

than

look

a spherical

like? shape,

If it has how

does

around?

really

is.

A~~~o~~~e~ge~enis-We thank H. Rosenbauer for data support and G. Monecke for the assistance in transfering the data from the UNIVAC to the Apple computer.

REFERENCES

Bame, S., Asbridge, J.. Feldman, W., Gosling, J. and Zwickl, R. (1981) Bi-directional streaming of solar wind electrons > 80 eV : ISEE evidence for a closed-field structure within the driver gas of an interplanetary shock. Geophys. Res. Letr. 8, 173.

Montgomery, M., Asbridge, J., Bame, S. and Feldman, W. (1974) Solar wind electron temperature depression followine some interolanetarv shock waves: evidence for magn&c rnerging?‘l. geo&ys. Res. 79, 3 103. Sanderson, 1.. Marsden, R., Rheinhard, R., Wenzel, K. and Smith, E. (1983) Geoplzys. Rex Letl. 10, 916. Schwenn, R., Muehlhaeuser, K., Marsch. E. and Rosenbauer, H. (1978) Two states of the solar wind at the time of solar activity minimum, II. Radial gradients of plasma parameters in the fast and slow streams, in Solar Wind Four (Edited by Rosenbauer, H.), p. 118. Sturrock, P. and Hartle, R. (1966) Two-fluid model of the solar wind. Phys. Rev. Left. 16, 628. Wilson, R. and Hildner. E. (1984) . , Are internlanetarv mag-? netic clouds manifestations of coronal transients at 1 a.~. Sob Phys. 91, 169.