Amajority large part of the earth’s magnetic field of earth (also known as the geomagnetic field) evoleshas its origin in the interior of the earth. The behavior of the geomagmetic field—and, by extension, the mechanisms of its origin—can only be understood by observing Observations of magnetic field changes onat and above the surface of the earth provides the only means of understanding the behaviour of the field, and hence of inference mechanisms of origin.

The sSurface [Editor1]field variations have different time- scales. Shorter- period variations, with periods of the order of a few days or less[Editor2], are attributed to electric currents flowing in the ionosphere and magnetosphere. The study of these variations is helpful in understanding the behavior and temporal development of current systems in the ionosphere and magnetosphere, and hence, of the solar-terrestrial environment. Longer- period variations, with periods of the order of years and decades[Editor3], are attributed to changes insidein the interior of the earth. In addition, Tthere are also field variations with periods longer than a few days[Editor4]; which have their origin in Sun such variations are caused by solar activities[Editor5]. These activities include the 27-day solar rotation period, the 11-year sunspot cycle of 11 years, and the 22-year solar magnetic cycle of 22 years. The vVariationsof the field over a with periods of the order of several years is are referred to as secular variations. The study of the shorter period variations is helpful in understanding the behaviour and time-development of current systems in the ionosphere and magnetosphere, hence of the Solar-Terrestrial environment.[Editor6]

The Ssun emits a continuousing stream of ionized particles,that which is known asthe solar wind. The interaction of this stream andwith the geomagnetic field results in the formation of a magnetized cavity in the immediate neighbourhood of that surrounds the earth[Editor7], called . This cavity is the magnetosphere.and is shaped The cavity is compressed on the dayside and stretched into an elongatedion tail on the nightside. The magnetic field lines of the earth contains open field lines in tThe polar flanks regions of the magnetosphere contain open magnetic field lines that originate from the earth’s surface rooted in the ground.[Editor8]The other end of these lines is connected to the interplanetary magnetic field (IMF) as a result of the interaction between the two earth’s fields and the IMF.

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[Editor1] I have started a new paragraph here: The first paragraph provides general information, while this paragraph focuses on surface field variations.

[Editor2]Consider stating the range more specifically, for example, “with periods of the order of 1–3 days.”

[Editor3]This is slightly confusing,since secular variations also have periods of the order of several years. I suggest the period be mentioned explicitly at this instance to avoid confusion between the time scales of long-period and secular variations.

[Editor4]Do these variations have a name? Or are these the secular variations mentioned below? Further, what is the time-scale range of these variations: between a few days and several years? Please clarify.

In this paragraph, the delineation between the time scales of the variations is not very clear. Please be more specific about the periods so that the different time scales are clearer.

[Editor5]The phrase “have their origin in the sun” is slightly vague. I have edited this part to make it more specific. Please check.

[Editor6]1For better flow, we have shifted this sentence to the part of the paragraph where short-period variations are discussed.

[Editor7]I have edited this according to my understanding of the definition of magnetosphere. Please check.

[Editor8]The phrase “magnetic field lines contains open field lines” was confusing. I have attempted to use conventional terminology in the edit. Please check whether the edit conveys the intended meaning.