Vanishing NPH and the Perimeter of the NPC

Forthcoming 2007/2008 Mars
(5)

Vanishing NPH and the Perimeter of the NPC

CMO #329 (25 March 2007)

Masatsugu MINAMI

1° In this issue, we shall give a forecast of a trend of the northern polar region (npr) at around the spring equinox of the northern hemisphere λ=360°Ls=000°Ls based on the observations in 1992. The north polar cap (npc) can be expected to attain the largest size just before the spring equinox, though it may difficult to see the details since at the same time it must be still covered by the north polar hood (nph). After the spring equinox the npc will show up its rigid state while still a thinner part of the nph will haunt the outside of the npc to conceal perimeter. In general it is hard for us to meet with the case where the Sub-Earth latitude DE=φ is appropriate to watch the npr fully, and since Ds=0° the npc does not easily shine up to us. Furthermore for the Japanese observers the appropriate case occurs in winter when the weather condition is dismal.

2° The diagrams presented in Coming (3) of CMO #327 p0541 show well that the spring equinox visited in 2005 too late (when δ was down to 9.7") and the tilt was in a poor condition (φ=17°S). Furthermore the terrestrial day of 22 Jan implies a period of hard days for the Japanese observers and in fact on the day just only four observations were reported in the CMO Gallery (from the US and England). On the contrary, in the coming 2007 Martian spring equinox, the condition will turns out to be better: The angular diameter is nearly maximal up to δ=15.7" (on 10 Dec 2007) and the tilt of the north pole will face towards us, though still discontent since it’s shallow (φ=4°N), and one month later (on 10 Jan 2008 when λ=015°Ls), it will decrease to φ=1.4°S when we need to watch the perimeter well. However before the equinox, for instance at λ=340°Ls (on 11 Nov), φ and δ reads 9°N and 12.2" respectively, and hence we can expect to be able to watch some interesting pre-equinox phenomena at the npr.

3°　As is also shown on the diagrams in Coming (3), the apparition in 1975 was more akin to the 2007 apparition. In 1975 the spring equinox occurred on 21 Dec 1975 when δ=16.1". But the condition was not better since φ=4°S, and was increasing further to the south. As for the present writer (Mn), he was not productive in 1975 partially because of the weather conditions. In 2005, the sky was utterly destructive in Dec 2005 in Fukui, and so we are afraid lest it should repeat again in Dec 2007.

In the 1990/1991 apparition where Mars most approached on 20 Nov 1990, the Martian spring equinox occurred on 4 Jan 1991 when δ=13.4" , while φ=13°S, and so it was as worse as in 2005.

4°　In this respect the 1992/93 apparition was quite better. The spring equinox visited earlier on 22 Nov 1992 when the weather was still stable in Fukui (while closest approach on 3 Jan 1993): Though δ was only 11.9", the tilt φ went up to 13°N and stayed more than 10°N around then so that the northern hemisphere was largely watched. At the time of λ=020°Ls, φ was still 9°N. So here we try to look back the case in 1992, and we expect it may be instructive to the observations in 2007 since the 2007 curves are quite near the 1992 case.　　

5°　In Coming (3), we touched on the Dawes slit inside the nph observed in 1990 around λ=320°Ls (while φ=4°S). Since it was apparent in 1990, we might have been able to observe it in 1992, but at least the present writer (Mn) did not catch it in the 1992 apparition. At λ=320°Ls, the diameter δ was 7.2", and even at the season λ=337°Ls when DAWES was considered to have observed the slit really in 1864, the diameter was mere δ=8.6" in 1992. Of course it might have been possible to detect it later: We observed the CM passing of M Acidalium from Japan from the beginning of November to mid-Nov at around λ=350°Ls~356°Ls with φ=12°N~13°N and δ=10.4"à11.2", but could not detect the slit. On 10 Nov 1992 (λ=354°Ls, φ=13°N, δ=10.8", ι=36°) at ω=010°W, 022°W, the northern part of M Acidalium was seen obscurely through the nph, but the boundary of the nph itself was also vague, and so the nph had become weaker. But on 13 Nov 1992 (λ=356°Ls, φ=13°N, δ=11.1") at younger ω=320°W, 330°W, 340°W, 349°W, the nph was thick and active over the morning M Acidalium, but we could not chase any more (already 4:30 JST at ω=359°W).

Back to 1990, we just note that, as repeatedly stated, the slit was visible from Japan around λ=320°Ls when δ=15.8". At the same season as DAWES himself detected the slit, the diameter was δ=18.1" in 1990 when the plane was nearly closest to the Earth, and M Acidalium was seen around 20h GMT (maybe from Europe).

6° In 1992, we thus failed to see the Dawes slit, but we were able to see similar slit or segment at a different region from around 24 Nov 1992 (λ=001°Ls, δ=12.1"). At around ω=190°W, Propontis I became clearly passing the CM, and its north was thinly covered by the nph, while seen through the weak hood, a dark segment became visible. The activity of the thin nph was not stable and so the segment appeared different from day to day. We here show three full drawings which were secured from 28 Nov (λ=003°Ls, δ=12.5") to 30 Nov 1992 (λ=004°Ls, δ=12.7") at ω=168°W~188°W. A more table of the nph activity on those days was once given in CMO #125 p1134-1135. In Internet, the Web site of the Fukui City Museum of Natural History shows in

http://www.nature.museum.city.fukui.fukui.jp/shuppan/kenpou/42/42-1-18.pdf 　

(see p8). The dark segment or line must have been the perimeter located at Gyndes of the solid npc. This was apparent to the naked eyes, while the nph was still present though thinner, and not clear over the npc. At that time we had already received several Lynxx ccd images of Don PARKER (DPk) made on 13 Nov 1992 (λ=355°Ls, δ=11.1", φ=13°N) at ω=190°W, 196°W, 209°W, and also we received Video images from Y HIGA (Hg) on 22 Nov at ω=255°W, and also several TP images from Y MOTITA (Mo) on 27 Nov and so on.

7° So next we turn to the question how the situation could be expected at around the Martian vernal equinox in 2007. The angular diameter will be satisfactory as often repeated. Just the tilt φ is shallower at 4°N~0°N during λ=000°Ls~010°Ls. This may be slightly worse than the case in 1992, but it can fully be expected that the interrelation of the vanishing nph with the dark perimeter of the rigid npc can be seen in an interesting manner. In Japan the area of the north of Propontis I will be watched by us (if the weather condition is not as worse as in 2005), and the region of the north of M Acidalium will face towards Europe.

In 1975, the present writer watched ω=198°W and others under a preferable seeing condition on 2 Jan 1975 (λ=006°Ls, δ=15.0"), but φ=5°S so that the npr was quite dull, even the boundary of the nph being unclear.

8° N.B. Personally to the present writer Mn who works here at Fukui, it is inconvenient that the spring equinox in 2007 occurs in December due to the expected dismal weather, while in 1992 it came in November. If we regard the observation rate in November 1992 as 100, the observations in December 1992 amounted 74. However in 2005, if we regard the observations in November 2006 as 100%, the rate in December 2005 was no more than 6%. In January, 90% in 1993, but in 2006 just 25%. They said the winter in 2006/2007 was warmer, but we have no statistics about the night skies at present. We are sure this is a particular situation at the areas facing to the north, and the situation at the region facing to the Pacific Ocean is different, and we expect good observations on the latter side of the Japanese island.

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