Interball contribution to the high-altitude cusp observations

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Planetary and Space Science 55 (2007) 2286–2294 www.elsevier.com/locate/pss

Interball contribution to the high-altitude cusp observations Z. Neˇmecˇeka,, J. Sˇafra´nkova´a, J. Meˇrkab, J. Sˇimu˚nekc, L. Prˇ echa a

Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, 180 00 Praha 8, Czech Republic b NASA Goddard Space Flight Center, Greenbelt, MD, USA c Institute of Atmospheric Physics, Czech Academy of Sciences, Bocni II, 141 31 Praha 4, Czech Republic Accepted 9 May 2007 Available online 10 August 2007

Abstract The polar cusps have traditionally been described as narrow funnel-shaped regions of magnetospheric magnetic field lines directly connected to magnetosheath, allowing the magnetosheath plasma to precipitate into the ionosphere. However, recent observations and theoretical considerations revealed that the formation of the cusp cannot be treated separately from the processes along the whole dayside magnetopause and that the plasma in regions like cleft or low-latitude boundary layer is of the same origin. Our review of statistical results as well as numerous case studies identified the anti-parallel merging at the magnetopause as the principal source of the magnetosheath plasma in all altitudes. Since effective merging requires a low plasma speed at the reconnection spot, we have found that the magnetopause shape and especially its indentation at the outer cusp is a very important part of the whole process. The plasma is slowed down in this indentation and arising multiscale turbulent processes enhance the reconnection rate. r 2007 Elsevier Ltd. All rights reserved. PACS: 94.30.Va; 94.30.Di Keywords: Cusp; Plasma mantle; Magnetopause; Magnetosheath; Magnetic reconnection; Low-latitude boundary layer

1. Introduction Although the low- and mid-altitude cusp precipitations were intensively investigated in course of last three decades, our knowledge on a structure of the cusp region in high altitudes is still under discussion. The Interball-1/Magion-4 satellite pair, called Interball-Tail project, was launched onto a highly elliptical orbit with 63 inclination, with apogee at 200 000 km or 31 RE and perigee at 750 km. During 2 years, both satellites performed two-point measurements in the high-altitude cusp reaching altitudes above those of the apogee of Polar (9 RE ). Two closely separated satellites could remain in the cusp region for periods of more than 2 h and scanned the vertical profile of the cusp from altitude 3RE up to the magnetosheath. Their simultaneous measurements are suitable for a study of the topology and dynamics of the high-altitude cusp and surrounding magnetosheath regions under various IMF Corresponding author. Tel.: +420 2 2191 2301; fax: +420 2 8468 5095.

E-mail address: [email protected] (Z. Neˇmecˇek). 0032-0633/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2007.05.021

orientations and solar wind conditions. The illustration in Fig. 1 presents projections of a part of the Magion-4 satellite trajectory onto GSM (geocentric solar magnetic) planes complemented with magnetic lines computed according to the Tsyganenko and Stern (1996) model and demonstrates the trajectory advantage. The abscissas show the 5-min averages of running magnetic field vectors as measured by Magion-4. One can note a good agreement of model and measured magnetic field directions in low latitudes. On the other hand, the model should be taken with care in high altitudes where the difference between measured and model magnetic field directions is notable. The cusp-like plasma was observed continuously nearly along the depicted orbit as can be seen from Fig. 2, and thus we can point out that the high-altitude cusp is far away from the general understanding of cusp as a narrow funnel-shaped region. The ion energy spectrograms recorded by three channels of the MPS/SPS device (Magion-4) are shown in three bottom panels of Fig. 2. The ion fluxes derived from Faraday cups onboard Interball-1 and Magion-4 are given

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Fig. 1. Projections of the Magion-4 orbit onto the X –Z (left part) and Y –Z (right part) planes according to the Tsyganenko and Stern (1996) model. The abscissas show the magnetic field vector as measured by Magion-4 each 5 min. Adapted from Neˇmecˇek et al. (2004).

in two top panels for reference. Ion flux profiles measured by two spacecrafts moving along the same orbit (Interball1 advanced Magion-4 by 70 min) exhibit some simultaneous changes as increase in the ion flux at 0747 or 0815 UT which can be attributed to the changes of the magnetosheath density above the cusp. However, these changes are small and do not mask the similarity of overall profiles. This similarity suggests that Magion-4 crossed a steady region and we can believe that the observed changes are of a spatial nature. The determination of the magnetopause crossing is very difficult but the most probable candidate is the change of the magnetic field (third and fourth panels in Fig. 2) accompanied with an increase of the ion flux at 0827 UT. Moreover, these changes correlate with changes in ion energy spectra shown in the last three panels. The spectra were recorded by the analyzers with different view angles. The I0 channel registers tailward streaming ions, whereas I180 channel (bottom panel) measures ions proceeding toward the Sun. These two analyzers are oriented roughly along the satellite spin axis and thus their data do not exhibit any distinct spin modulation. On the other hand, the I90 analyzer is oriented nearly perpendicularly to the spin axis and scans a broad range of pitch angles during one satellite revolution. The classifications of visited regions is given below the figure for the sake of reference only because we will discuss their definition in the next section. The analysis of the set of cusp observations during the years 1995–1997 presented in many related papers has shown that the cusp is well defined and persistent at altitudes of 4213 RE (Sandahl et al., 1997, 2000; Sandahl, 2002). Nevertheless, the region above 8210 RE is a highly turbulent (Savin et al., 1998, 2001, 2002; Meˇrka et al., 2000, 2002) and the cusp plasma is mixed with the magnetosheath population (Savin et al., 2004) at these altitudes. Thus, the dimensions of the cusp region are variable and controlled mainly by the interplanetary (magnetosheath) magnetic field orientation. In this overview of Interball project observations, the survey of some results regarding the high-altitude cusp region is presented. Our attention was predominantly concentrated on following topics:

(1) definition of the high-altitude cusp and related regions, (2) average location of the cusp, (3) influence of the tilt angle of the Earth’s dipole, and (4) the magnetopause in the cusp region. The limited extend of the paper cannot touch all peculiarities of the cusp formation and thus it is concentrated on the cusp geometry. The cusp in a broader sense and influence of the cusp processes on the morphology of the whole magnetosphere and its formation are treated in Neˇmecˇek and Sˇafra´nkova´ (2007).

2. Cusp definition The traditional image of the cusp is a narrow region where magnetosheath plasma can directly enter into the magnetosphere and the dayside ionosphere (Haerendel et al., 1978; Smith and Lockwood, 1996). The particle composition and charge are the same as those found in the solar wind. Over a decade after initial cusp observation, the precise definition of this region was developed from observations of low-altitude ion and electron precipitations (Newell and Meng, 1988). These investigations have been extensively provided by the Defense Meteorological Satellite Program (DMSP) and using 12; 000 crossings, Newell and Meng (1988) developed a simple and practical definition of the cusp and cleft/low-latitude boundary layer (LLBL) regions. If E e o200 eV, E i o2700 eV, j i 4 1010 eV=cm2 .s.sr, and j e 46  1010 eV=cm2 .s.sr, the region is called the cusp (E i and E e are the average ion and electron energies, j i and j e are the energy fluxes of ions and electrons, respectively). If either 3000 eVoE i o6000 eV or 220 eVoE e o600 eV, the region was identified as cleft/ LLBL. The cusp (or cusp proper) and the cleft/LLBL were distinguished later from the mantle (Newell et al., 1991a, b). The identification of the mantle at low altitudes is based on the following criteria (Newell et al., 1991a): (1) location—immediately poleward of the dayside oval; and (2) the associated soft spectra of the ions which have densities from a few times 10 2 to a few times 10 1 cm 3 , and a temperature range from a few tens of eV to about 200 eV.

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Fig. 2. Magion-4 observations of the precipitating ions. The first panel shows the Interball-1 ion flux for the sake of reference. The second panel presents the Magion-4 ion flux and next three panels depict ion energy spectra measured in three directions. The magnetopause crossing is distinguished by a vertical line, the identification of crossed regions is labeled in the bottom part. Adapted from Neˇmecˇek et al. (2004).

At the low altitudes, the cusp can be observed in local magnetic time extents up to 3.7 h of MLT (Maynard et al., 1997). However, the low- and mid-altitude cusp is quite narrow in latitude, only about 122 (Newell et al., 1989, 1991a; Aparicio et al., 1991; Potemra et al., 1992). On the other hand, at high altitudes, the cusp-like plasma occupies a wide range of magnetic latitudes as well as a broader region of longitudes (Aparicio et al., 1991; Newell and Meng, 1994; Maynard et al., 1997). Fig. 3 depicts, in the GSM coordinates, all parts of the Magion-4 orbits on which the cusp-like plasma was detected. It can be seen that the cusp occupies a significant part of high-latitude magnetosphere.

The actual cusp location depends on magnetospheric, solar wind, and IMF conditions and these effects certainly contribute to the cusp width as displayed in Fig. 3. However, the whole region is highly turbulent, the amplitudes of fluctuations of all parameters exceed their mean values (Savin et al., 2002, 2004). The waves launching from this region can significantly modify the particle distribution on their path to the ionosphere. The other possible source of such modification can be field aligned potentials at the cusp boundaries. The evolution of the distribution function would lead to a different classification of the plasma regime in lower altitudes.

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Fig. 3. Cusp transitions observed by Magion-4 during the 1996 and 1997 years (in GSM coordinates). Different colors distinguish measurements made closer or farther than 3 RE from the magnetopause (the Tsyganenko and Stern (1996) model). Adapted from Meˇrka et al. (2002).

Several mechanisms of magnetosheath plasma entry into the magnetosphere have been suggested.However, as an extensive discussion in Sibeck et al. (1999) has shown, observational facts are generally consistent with magnetic reconnection being a dominant source of the cusp plasma, whereas other mechanisms can contribute to the cusp population under specific circumstances. As noted there, a unique feature of reconnection is that it requires the relevant physical processes to take place only in a narrow diffusion region, while its consequences are global: Once the interplanetary and magnetospheric lines become interconnected, they remain connected while being convected with the solar wind, and plasma continues to enter the magnetosphere. This is in contrast to all other mechanisms that operate only locally and their occurrence at different locations is essentially uncorrelated. We can distinguish two basic reconnection processes: subsolar magnetopause reconnection during southward IMF (Dungey, 1961; Reiff et al., 1977) and lobe reconnection during northward IMF (Luhmann et al., 1984). If reconnection takes place equatorward of the cusp, the convective flow is antisunward and magnetospheric open field lines are moved poleward through the cusps. The process is schematically shown in Fig. 4a. It is a classical drawing but we would like to point out several peculiarities. The whole dayside magnetopause is open and the definition of the magnetopause as the last field line connected to Earth fails. The magnetopause should be probably placed through kinks of the magnetic field lines that are in permanent motion tailward. Magnetic field minima at the cusps lead to indentations of the magnetopause. Such indentation results from the pressure balance at the magnetopause (e.g., Sotirelis and Meng, 1999) but experimental evidence of it is one of achievements of the Interball project and we will discuss it later. Subsolar reconnection results in several plasma regimes that are shown in Fig. 4b. The magnetosheath plasma enters the dayside magnetopause layer; it is slightly

accelerated by magnetic tension and precipitates at the equatorward side of the region (Fedorov et al., 2000). The magnetopause part of this region is often called an entry layer, whereas the magnetospheric part is known as a cleft. Since this region extends to low latitudes on the magnetopause flanks, the name LLBL is now preferred. The magnetic field lines enriched with plasma convects tailward or, in other words, the plasma undergoes a ~ B ~ drift and thus it appears on the magnetic tailward E field lines that are bent tailward. Since the magnetic mirror at lower altitudes reflects a significant portion of the plasma, an upward flow can be observed in the poleward part of the region. This region is usually called the plasma mantle. The very simplified description that is shown by arrows in Fig. 4b expects a low-temperature or monoenergetic plasma entering at the subsolar point. As we noted above, nearly the whole dayside magnetopause is open and entering plasma is rather hot. It leads to a spread of pitch-angles of entering particles that are then mirrored at different altitudes. These two effects lead to the creation of a broad region occupied by particles moving in both directions that are mixed by turbulent processes. This region is called the cusp and it is distinguished by a vertical hatching in Fig. 4b. If the reconnection site is located poleward of the cusp (Fig. 5a), the resulting convective flow is sunward and an open field line is pushed equatorward through the cusp in this case. The convective flow is slow because it opposes the fast magnetosheath bulk flow. The subsolar magnetopause position and the cusp latitude are not sensitive to reconnection rate variations during northward IMF (Newell et al., 1989; Palmroth et al., 2001; Sˇafra´nkova´ et al., 2002b). The magnetic field lines reconnected at one hemisphere can re-reconnect tailward of the conjugated cusp and close the magnetosheath plasma on dayside magnetospheric field lines. On the other hand, the tail part of the reconnected lobe line is decoupled from the Earth and becomes an IMF

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Fig. 4. The cusp during a prolonged interval of southward pointing IMF. (a) Configuration of the magnetic field lines; estimated location of the magnetopause is shown as the heavy dashed line. (b) Schematics of plasma flows in the cusp and adjacent regions; the dominating flow directions in different regions are shown by arrows.

Fig. 5. The cusp during a prolonged interval of northward pointing IMF. (a) Configuration of the magnetic field lines; estimated location of the magnetopause is shown as the heavy dashed line. (b) Schematics of plasma flows in the cusp and adjacent regions; the dominating flow directions in different regions are shown by arrows.

line. This process leads to the creation of a layer of tailward flowing plasma adjacent to the night-side highlatitude magnetopause that is called reconnection layer in Sˇafra´nkova´ et al. (1998) (Fig. 5b). This layer has similar properties as the plasma mantle that can be found at the same location during southward IMF intervals but there are two principal differences: the reconnection layer is outside the magnetopause and it is supplied with plasma directly from the magnetosheath. The plasma flows are shown in Fig. 5b. If dayside (LLBL) lines are closed the particles are bouncing and they convect with the magnetic field to the nightside. The presence of horizontal IMF components can prevent rereconnection and the dayside LLBL is on open field lines in such a case. A broad region equatorward of the entry point is filled with the heated plasma due to its convection equatorward, magnetic mirroring, and turbulence excited by the counterstreaming flows. 3. High-altitude cusp location The behavior of the plasma and magnetic field in the high-altitude cusp differs from those observed in lower

altitudes because the high-altitude cusp represents an interface between two regimes. The energy connected with the plasma bulk flow dominates in the magnetosheath, whereas the energy associated with the Earth’s magnetic field determines the plasma properties at lower altitudes. Ion and electron distributions undergo a significant evolution in course of reconnection (Fedorov et al., 2002) and further on the path from the outer cusp to the ionosphere due to time-of-flight effect (Lockwood et al., 1995). Studies of Meˇrka et al. (1999, 2000, 2002) and Neˇmecˇek et al. (2000) were based on particle precipitation patterns derived from the measurements of the MPS/SPS ion/ electron spectrometers onboard the Magion-4 satellite (Neˇmecˇek et al., 1997). The classification of a cusp precipitation was based on observations of the differential energy flux which should peak at an energy lower than 200 eV for electrons and 1 keV for ions. This is consistent with criteria of other authors (e.g., Kremser and Lundin, 1990; Newell and Meng, 1988; Newell et al. 1991a, b; Aparicio et al., 1991). To exclude the magnetosheath, an additional restriction on the bulk velocity to be less than 100 km/s or field aligned was applied. An example of

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Magion-4 observations is shown in Fig. 2. Following the above discussed view of the cusp, we do not distinguish between the cusp and mantle in a further analysis. To compare Magion-4 observations with those from lowaltitudes, they have taken all parts of orbits on which the cusp-like plasma has been observed and projected them to the DMSP altitudes (835 km) using the Tsyganenko and Stern (1996) model of the magnetospheric magnetic field. The tilt angle is defined as the angle between the Earth’s dipole axis and the Z GSM coordinate. Zhou and Russell (1997) used magnetopause crossings observed by Hawkeye to demonstrate that, in the neighborhood of the magnetopause, the polar cusp location is controlled by the tilt of the dipole, so that it moves increasingly toward the Sun as the dipole tilts sunward. This trend seems to be opposite to that determined from low-altitude observations (e.g., Newell and Meng, 1989). However, it should be noted that in this study the cusp position was derived statistically from observations of the magnetopause crossings below and above the cusp, not from direct observations of the cusp itself. It means that it reflects to the location of the cusp entry, whereas all other studies describe the location of its footprint. Zhou et al. (1999) demonstrated the dipole tilt angle effect on the cusp location using mid-altitude data from POLAR. The authors concluded that the position of the cusp depends significantly on the dipole tilt angle. When the dipole tilts more toward the Sun, the cusp moves more poleward to higher invariant latitudes, roughly 1 for every 14 of tilt. Neˇmecˇek et al. (2000) completed previous studies of the cusp latitudinal position with results obtained in the vicinity of the magnetopause (i.e., altitudes in range from 10 to 15 RE ). Figs. 6a and b show the latitudinal position of the 1-min parts of orbits from Fig. 3 as a function of the dipole tilt. The sign of the tilt was chosen to be positive during a summer time in the northern hemisphere. Each scatterplot is fitted with a linear function and parameters describing this function (mean cusp

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location for 0 of tilt and a corresponding slope) are given on the top of figures. A significant portion of our observations was made near the magnetopause and thus the data set was divided into two parts: observations closer than 3 RE to the magnetopause and more distant observations (43 RE ). The model magnetopause for these estimations was taken from the Tsyganenko and Stern (1996) model. The results of this selection according to Neˇmecˇek et al. (2000) are plotted in two panels of Fig. 6. The authors noted that the slope of the fitted function is 0.12 for the first group of observations (Fig. 6a) and it is very similar to that found by Zhou et al. (1999). Observations in the vicinity of the magnetopause (o3 RE ) plotted in Fig. 6b suggest that the cusp location is more sensitive to the dipole tilt at these altitudes (the slope of the fitted function is 0.16). A similar analysis of a possible influence of other parameters on the cusp location (IMF components, upstream pressure) has shown that the tilt angle dependence is the most striking and biases all other plots. For this reason, Meˇrka et al. (1999) have corrected locations of all cusp observations using the linear functions given in Fig. 6. Since the coverage of the cusp region with observations was not uniform (see Fig. 3), they have divided the target surface into bins 0:5 of MLAT by 0.5 h of MLT and computed the probability of cusp observation dividing the number of minutes of cusp observations by the total number of minutes that the spacecraft spent in each particular bin. The resulting plot is shown in Fig. 7 and reveals that: (1) the low-altitude projection of the highaltitude cusp occupies a much broader region than inferred from low-altitude observations, (2) the probability of cusp observations exhibits two maxima—one at morning and the other at afternoon local times instead of one maximum at local noon that is generally expected, and (3) the probability of the cusp observation in a particular place is rather low. The last point suggests that the cusp is narrower than its projection in Fig. 7 but the displacement

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correction of the Petrinec and Russell (1996) model. The results of this correction are surprisingly good and can be briefly summed up as follows:

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The high-latitude magnetopause cross-section is flattened. The location of this depression is controlled by the tilt angle of the Earth’s dipole. A most probable source of the depression is the magnetospheric cusp. The data set shows a similar depression for both signs of the tilt. The indentation can be observed at geomagnetic latitudes higher than 30 and in a broad range of X GSE coordinates ( 2RE pX GSE p8RE ). An averaged deepness is 2:5 RE but the magnetopause was often observed 4 RE earthward of the expected location. The position and deepness of the depression is in qualitative agreement with the Sotirelis and Meng (1999) model.

20 0 Fig. 7. Probability of Magion-4 cusp observations in high altitudes projected to 835 km of altitude along the model field lines. The correction on the tilt angle dependence of the cusp location was taken into account. Adapted from Meˇrka et al. (1999).

of this region due to changes of the IMF direction causes an apparent broadening of its projection in Fig. 7. These effects are broadly discussed in Meˇrka et al. (2002). They have found that changes of IMF BZ shift the cusp in latitude whereas the presence of a strong IMF BY leads to observations of two cusps in different local times. 4. Magnetopause indentation and its influence on cusp processes Sˇafra´nkova´ et al. (2002a) used a large data set including low-latitude as well as high-latitude magnetopause crossings and tested several recent empirical magnetopause models and concluded that (1) the difference between investigated models is smaller than the error of prediction caused by the factors not included in models, (2) the dayside magnetopause is indented in the cusp region, (3) the deepness of the indentation can reach 44RE and its position and deepness are in a qualitative agreement with the Sotirelis and Meng (1999) model, (4) the dimensions of the indentation do not depend on the dipole tilt, whereas its location does, and (5) the location and/or extent of the indentation seems to be controlled by the IMF BZ component. In order to account for the magnetopause indentation, Sˇafra´nkova´ et al. (2005) plotted the normalized distance of the high-latitude crossings from the Earth’s center as a function of the X coordinate separately for positive and negative tilts. The resulting functions were fitted with curves describing the dependence of the magnetopause location on the tilt angle and these curves were used for a

The presence of the magnetopause indentation has important consequences for lobe reconnection and formation of the high-altitude cusp during periods of the northward IMF. Fig. 8 shows the plasma parameters measured by Interball-1 inside this indentation (Savin et al., 1998). The satellite moved through the northern lobe (until 0420 UT) and then through the cusp, crossed the magnetopause at 0427 UT and entered a disturbed region characterized by a very low plasma velocity. The vX component is minor and changes from positive to negative values. The largest component is positive vZ . It suggests a nearly vertical flow inside the magnetopause indentation. At about 0517 UT, the satellite enters the free-flow magnetosheath dominated by vX and vY components that correspond to the satellite location in the dusk highlatitude magnetosheath. The conditions inside the indentation (low velocity, large fluctuations) are favorable for reconnection and it can explain the fact that the cusp precipitation is often very intensive during periods of northward IMF. Otherwise, without deceleration of the magnetosheath plasma inside the indentation, the flow velocity would be supersonic above the cusp and the precipitation flow would be very low (Neˇmecˇek et al., 2003). Properties of the turbulence on both sides of the magnetopause inside the cusp indentation were intensively studied in Savin et al. (2002, 2004) and it was shown that a significant portion of the kinetic energy of the plasma flow is converted into wave energy. This turbulence exhibits multi-scale properties and thus, large-scale structures can be identified inside this indentation. An example of such vortex-like structure is described in Sˇafra´nkova´ et al. (2002b). We would like to note that this structure was observed by two spacecraft with a delay of about 20 min. This suggests that these patterns can be steady. The diameter of this structure estimated by Sˇafra´nkova´ et al. (2002b) from two-point measurements was about 2000 km and thus such structure can be observed rarely

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even if it is a regular feature of the high-latitude magnetopause. 5. Conclusion In the present paper, we have tried to show that 5 years of Interball project observations contributed significantly to our understanding of processes that form the cusp and adjacent regions. We have attempted to put into context the results that were published separately by different groups in tens of papers in course of the last 10 years. Owing to Interball project measurements as well as to many papers based on Polar and Cluster observations, we are now able to draw pictures like Figs. 3 and 4 in this paper. Nevertheless, the way from this gross-scale and global structure to the deep understanding of details of cusp processes is still in front of us.

Acknowledgments The presented work was partly financially supported by the Research plan MSM 0021620860 that is financed by the Ministry of Education of the Czech Republic and partly by the Czech Grant Agency under Contracts 205/05/ 0170 and 205/06/0875. Their financial supports are greatly acknowledged. References Aparicio, B., Thelin, B., Lundin, R., 1991. The polar cusp from a particle point of view: a statistical study based on Viking data. J. Geophys. Res. 96 (A8), 14023–14031. Dungey, J.W., 1961. Interplanetary field and auroral zones. Phys. Rev. Lett. 6, 47. Fedorov, A., Dubinin, E., Song, P., Budnick, E., Larson, P., Sauvaud, J.-A., 2000. Characteristics of the exterior cusp for steady southward

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