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Evolution of the coronal plasma sheet from cycle maximum to cycle minimum observed at meter wavelengths
0273-I 177(95)00332-O
A& @ace Rrs. Vol. 16. No. 9, pp. (9)185-(9)188, 1995 Cqyight @Z1995 COSPAR Printed in Great Britain. All rights reserved. 0273-l 177/95 $9.50 + 0.00
EVOLUTION OF THE CORONAL PLASMA SHEET FROM CYCLE MAXIMUM TO CYCLE MINIMUM OBSERVED AT METER WAVELENGTHS P. Lantos* and C. E. Alissandrakis** * Observatoire de Paris, 92195 Meudon, France ** National University of Athens, 15784 Athens, Greece
ABSTRACT The Nansay Radioheliograph provides maps of the quiet solar corona around 160 MHz. In addition to coronal holes, localized emission sources and quiet sun, an unresolved component can be detected on the disk. The so-called “coronal plateau” corresponds to the K-corona dense regions observed on the limb and is thus located at the base of the interplanetary plasma sheet. Its evolution is studied from 1980 to 1986, during the declining phase of the cycle 21. INTRODUCTION The Heliospheric Current Sheet (HCS) is a basic structure of the interplanetary medium, both as a magnetic sector boundary and as a region of slow solar winds /l/. With the associated corotating interaction regions it plays a role in the acceleration of low energy protons, in the propagation of solar protons and in the modulation of cosmic rays. In the low corona, the loops located at the basis of the HCS are detected with K-coronameters as density enhancements /2/; at higher altitudes, where coronal magnetic field measurements are not available, the extrapolation of photospheric magnetic fields predicts the shape of the and the interplanetary neutral line /3/. N evertheless, this extrapolation is model-dependent K-corona measurements have a rather low resolution in longitude, because of the integration of the limb emission along the line of sight. New low altitude observations of the coronal plasma sheet are thus important as a contribution to our understanding of the origin of the slow solar winds and of many interplanetary phenomena. OBSERVATIONS The Nancay Radioheliograph is a dedicated instrument providing high time resolution observations of the Sun in the East-West and North-South directions. Two-dimensional maps are calculated taking advantage of the rotation of the solar sxis in the frame of the instrument, provided that the brightness distribution remains sufficiently stable during the day. As the short duration bursts can be removed, the main limitation
(9)185
is the presence of intense,
(9)186
I’. Lantos and C. E. Alissandrakis
0 MHz
Figure 1. Map of the Sun obtained on June 9, 1984 at 169 MHz with the aperture
synthesis
method. time-varying,
long-duration
noise storms.
Figure 1 shows a map obtained
at 169 MHz on
June 9, 1984 in the declining phase of the sunspot cycle. Only a few localized emission sources are present, the brightest being at the center of the solar disk. The region shown in black, which corresponds to the isophote with brightness temperature 100 000 K below the lowest isophote of the emission sources (9x105 K), is the so-called “coronal plateau”. It has been shown /4,5/ that the coronal plateau is in good agreement with the shape and the location of the coronal plasma sheet seen on K-corona observations. Furthermore, as this observation is obtained on the disk, the resolution in longitude is better than that of the optical observations beyond the limb and thus the comparison structures is easier. EVOLUTION
OF THE CORONAL
PLATEAU
with underlying
DURING
chromospheric
THE CYCLE
From the daily radio maps, synoptic charts in heliographic longitude-latitude coordinates can be computed. Figure 2 shows a comparison of synoptic charts obtained during different phases of the solar cycle. In July 1980 (upper frame), during the sunspot cycle maximum, the coronal plateau (indicated in black) surrounds the equatorial holes (indicated with hatched contours). Missing days are due to the frequent presence of intense noise storms during this period of the cycle. During the declining phase of the cycle (middle frame), in June-July 1984, the structure of the coronal plateau is much simpler, in the absence of equatorial corond holes. The plateau oscillates from one hemisphere to the other avoiding the coronal holes seen here on the left side of the figure.
(9)187
Evolutiooof the Corooal Plasma Sheet
-lo b io do do 00
~bo~ioiio~Gol~~~02402~o2u03603iO3403
03 0 .e.b
Figure 2. Radio synoptic charts compared with the neutral line at 2.5 solar radii (heavy line) computed with the source surface model: l- upper frame: 169 MHz, Carrington rotation 1696 (June 1980) 2- middle frame: 169 MHz, Car&ton rotation 1750 (June-July 1984) 3- lower frame: 164 MHz, Carrington rotation 1776 (June 1986). Finally, during the minimum of the sunspot cycle (lower frame), in June 1986, the coronal plateau becomes almost equatorial and is only disturbed by a low latitude coronal hole. A comparison with the neutral line of the source surface field at 2.5 solar radii /3,6/ is given on the same figure. During solar cycle minimum (lower frame) the agreement is excellent: the extrapolated neutral line lies in the middle of the coronal plateau and passes through
P. Lantos and C. E. Atissnadrakis
(9)188
almost all the radio source maxima. During the declining phase, the general shape of both structures is similar but the computed neutral line, on the left side of the figure, is clearly at higher southern latitude than the observed coronal plateau. the shapes becomes
totally
Finally, during cycle maximum,
different from each other, the coronal plateau being double at
some longitudes because of the equatorial coronal hole. The difference between the extrapolated neutral line and the observed coronal plateau obviously altitudes are different, the estimated
radius and 2- the source surface does not actually discussed in /7/,
arises from two factors:
exist and the source surface model,
is only one of the possible models of photospheric
lation to high altitudes. dipole is doninant,
Nevertheless,
as
magnetic field extrapo-
during solar cycle minimum, when the solar magnetic
large scale magnetic
the coronal structures
l- the
altitude of radio emission being lower than 1.3 solar
fluxes in both hemispheres
are radial, the model gives excellent
are almost equal and
results when compared
to low
altitude observations. REFERENCES 1. A.J. Hundausen, An Interplanetary View of Coronal Holes, in: Coronal Holes and High Speed Wind Shwmas, ed. J.B. Zirker, Colorado Associated University Press, Boulder, p.225, 287., (1977). 2. K. Hakamada, Three-dimensional Wind Speed Distribution
Projected
Structure of the Coronal Magnetic Field and the Solar on the Photosphere
in 1974, J. Geophys.
Res., 92, A5,
4339-4348, (1987). 3. J.T. Hoeksema and P.H. Scherrer, The Solar Magnetic Field -1976 through 1985-, Report UAG-94, World Data Center A for Solar-Terrestrial Physics, Boulder, USA, (1986). 4. P. Lantos, C.E. Alissandrakis and D. Rigaud, Quiet-Sun Emission and Local Sources at Meter and Decimeter Wavelengths and their Relationship with the Coronal Neutral Sheet, Solar Physics, 137, 225-256, (1992). 5. P. Lantos and C.E. Alissandrakis, Large Scale Structure of the Solar Corona in the Declining Phase of the Solar Cycle, Space Science Reviews, in press, (1994). 6. J.T. Hoeksema, private communication 7. X. Zhao and J.T. Hoeksema, A Coronal Magnetic Field Model with Horizontal and Sheet Currents, Solar Physics, 151, 91-105, (1994).
Volume
A& @ace Rrs. Vol. 16. No. 9, pp. (9)185-(9)188, 1995 Cqyight @Z1995 COSPAR Printed in Great Britain. All rights reserved. 0273-l 177/95 $9.50 + 0.00
EVOLUTION OF THE CORONAL PLASMA SHEET FROM CYCLE MAXIMUM TO CYCLE MINIMUM OBSERVED AT METER WAVELENGTHS P. Lantos* and C. E. Alissandrakis** * Observatoire de Paris, 92195 Meudon, France ** National University of Athens, 15784 Athens, Greece
ABSTRACT The Nansay Radioheliograph provides maps of the quiet solar corona around 160 MHz. In addition to coronal holes, localized emission sources and quiet sun, an unresolved component can be detected on the disk. The so-called “coronal plateau” corresponds to the K-corona dense regions observed on the limb and is thus located at the base of the interplanetary plasma sheet. Its evolution is studied from 1980 to 1986, during the declining phase of the cycle 21. INTRODUCTION The Heliospheric Current Sheet (HCS) is a basic structure of the interplanetary medium, both as a magnetic sector boundary and as a region of slow solar winds /l/. With the associated corotating interaction regions it plays a role in the acceleration of low energy protons, in the propagation of solar protons and in the modulation of cosmic rays. In the low corona, the loops located at the basis of the HCS are detected with K-coronameters as density enhancements /2/; at higher altitudes, where coronal magnetic field measurements are not available, the extrapolation of photospheric magnetic fields predicts the shape of the and the interplanetary neutral line /3/. N evertheless, this extrapolation is model-dependent K-corona measurements have a rather low resolution in longitude, because of the integration of the limb emission along the line of sight. New low altitude observations of the coronal plasma sheet are thus important as a contribution to our understanding of the origin of the slow solar winds and of many interplanetary phenomena. OBSERVATIONS The Nancay Radioheliograph is a dedicated instrument providing high time resolution observations of the Sun in the East-West and North-South directions. Two-dimensional maps are calculated taking advantage of the rotation of the solar sxis in the frame of the instrument, provided that the brightness distribution remains sufficiently stable during the day. As the short duration bursts can be removed, the main limitation
(9)185
is the presence of intense,
(9)186
I’. Lantos and C. E. Alissandrakis
0 MHz
Figure 1. Map of the Sun obtained on June 9, 1984 at 169 MHz with the aperture
synthesis
method. time-varying,
long-duration
noise storms.
Figure 1 shows a map obtained
at 169 MHz on
June 9, 1984 in the declining phase of the sunspot cycle. Only a few localized emission sources are present, the brightest being at the center of the solar disk. The region shown in black, which corresponds to the isophote with brightness temperature 100 000 K below the lowest isophote of the emission sources (9x105 K), is the so-called “coronal plateau”. It has been shown /4,5/ that the coronal plateau is in good agreement with the shape and the location of the coronal plasma sheet seen on K-corona observations. Furthermore, as this observation is obtained on the disk, the resolution in longitude is better than that of the optical observations beyond the limb and thus the comparison structures is easier. EVOLUTION
OF THE CORONAL
PLATEAU
with underlying
DURING
chromospheric
THE CYCLE
From the daily radio maps, synoptic charts in heliographic longitude-latitude coordinates can be computed. Figure 2 shows a comparison of synoptic charts obtained during different phases of the solar cycle. In July 1980 (upper frame), during the sunspot cycle maximum, the coronal plateau (indicated in black) surrounds the equatorial holes (indicated with hatched contours). Missing days are due to the frequent presence of intense noise storms during this period of the cycle. During the declining phase of the cycle (middle frame), in June-July 1984, the structure of the coronal plateau is much simpler, in the absence of equatorial corond holes. The plateau oscillates from one hemisphere to the other avoiding the coronal holes seen here on the left side of the figure.
(9)187
Evolutiooof the Corooal Plasma Sheet
-lo b io do do 00
~bo~ioiio~Gol~~~02402~o2u03603iO3403
03 0 .e.b
Figure 2. Radio synoptic charts compared with the neutral line at 2.5 solar radii (heavy line) computed with the source surface model: l- upper frame: 169 MHz, Carrington rotation 1696 (June 1980) 2- middle frame: 169 MHz, Car&ton rotation 1750 (June-July 1984) 3- lower frame: 164 MHz, Carrington rotation 1776 (June 1986). Finally, during the minimum of the sunspot cycle (lower frame), in June 1986, the coronal plateau becomes almost equatorial and is only disturbed by a low latitude coronal hole. A comparison with the neutral line of the source surface field at 2.5 solar radii /3,6/ is given on the same figure. During solar cycle minimum (lower frame) the agreement is excellent: the extrapolated neutral line lies in the middle of the coronal plateau and passes through
P. Lantos and C. E. Atissnadrakis
(9)188
almost all the radio source maxima. During the declining phase, the general shape of both structures is similar but the computed neutral line, on the left side of the figure, is clearly at higher southern latitude than the observed coronal plateau. the shapes becomes
totally
Finally, during cycle maximum,
different from each other, the coronal plateau being double at
some longitudes because of the equatorial coronal hole. The difference between the extrapolated neutral line and the observed coronal plateau obviously altitudes are different, the estimated
radius and 2- the source surface does not actually discussed in /7/,
arises from two factors:
exist and the source surface model,
is only one of the possible models of photospheric
lation to high altitudes. dipole is doninant,
Nevertheless,
as
magnetic field extrapo-
during solar cycle minimum, when the solar magnetic
large scale magnetic
the coronal structures
l- the
altitude of radio emission being lower than 1.3 solar
fluxes in both hemispheres
are radial, the model gives excellent
are almost equal and
results when compared
to low
altitude observations. REFERENCES 1. A.J. Hundausen, An Interplanetary View of Coronal Holes, in: Coronal Holes and High Speed Wind Shwmas, ed. J.B. Zirker, Colorado Associated University Press, Boulder, p.225, 287., (1977). 2. K. Hakamada, Three-dimensional Wind Speed Distribution
Projected
Structure of the Coronal Magnetic Field and the Solar on the Photosphere
in 1974, J. Geophys.
Res., 92, A5,
4339-4348, (1987). 3. J.T. Hoeksema and P.H. Scherrer, The Solar Magnetic Field -1976 through 1985-, Report UAG-94, World Data Center A for Solar-Terrestrial Physics, Boulder, USA, (1986). 4. P. Lantos, C.E. Alissandrakis and D. Rigaud, Quiet-Sun Emission and Local Sources at Meter and Decimeter Wavelengths and their Relationship with the Coronal Neutral Sheet, Solar Physics, 137, 225-256, (1992). 5. P. Lantos and C.E. Alissandrakis, Large Scale Structure of the Solar Corona in the Declining Phase of the Solar Cycle, Space Science Reviews, in press, (1994). 6. J.T. Hoeksema, private communication 7. X. Zhao and J.T. Hoeksema, A Coronal Magnetic Field Model with Horizontal and Sheet Currents, Solar Physics, 151, 91-105, (1994).
Volume