Here,the direction of current is towards east as the positively charged alpha particles are moving towards west. The deflection is towards north, so the force is towards north, so, we are given that direction of current is towards west and direction of force is towards north.
Let us now hold the forefinger, middle finger and thumb of our left-hand at right angles to one another as per Fleming’s left hand rule. Adjust the hand in such a way that our mid finger points towards west (in the direction of current) and thumb points towards north (in the direction of force). Now, if we look at our forefinger, it will be pointing upward.So the magnetic field is in the upward direction because the direction of the forefinger gives the direction of the magnetic field.